One of the most important stages in metal complex catalysis - the interaction of the substrate Y with the complex - occurs according to three mechanisms:

a) Replacement of the ligand with a solvent. This stage is usually depicted as the dissociation of the complex

The essence of the process in most cases is the replacement of the ligand L with a solvent S, which is then easily replaced by a substrate molecule Y

b) Attachment of a new ligand along the free coordinate with the formation of an associate followed by dissociation of the replaced ligand

c) Synchronous substitution (type S N 2) without the formation of an intermediate

In the case of Pt (II) complexes, the reaction rate is very often described by the two-route equation

where k S and k Y- rate constants of the processes proceeding by reactions (5) (with solvent) and (6) with ligand Y. For example,

The last stage of the second route is the sum of three fast elementary stages - the cleavage of Cl -, the addition of Y, and the elimination of the H2O molecule.

In planar square complexes of transition metals, the trans effect, formulated by II Chernyaev, is observed - the effect of LT on the rate of substitution of the ligand in the trans position to the LT ligand. For Pt (II) complexes, the trans effect increases in the series of ligands:

H 2 O ~ NH 3

The presence of the kinetic trans effect and thermodynamic trans effect explains the possibility of the synthesis of inert isomeric complexes of Pt (NH 3) 2 Cl 2:

Coordinated ligand reactions

Reactions of electrophilic substitution (S E) of hydrogen by a metal in the coordination sphere of a metal and processes inverse to them

SH - H 2 O, ROH, RNH 2, RSH, ArH, RCCH.

Even H 2 and CH 4 molecules are involved in reactions of this type

Insertion reactions L by connection M-X

In the case of X = R (organometallic complex), metal-coordinated molecules are also incorporated into the M-R bond (L – CO, RNC, C 2 H 2, C 2 H 4, N 2, CO 2, O 2, etc.). The insertion reactions are the result of an intramolecular attack of a nucleophile X on a molecule coordinated by the-or-type. Reverse reactions - reactions of-and-elimination

Oxidative addition and reductive elimination reactions

M 2 (C 2 H 2)  M 2 4+ (C 2 H 2) 4–

Apparently, in these reactions there is always a preliminary coordination of the attached molecule, but this is not always possible to fix. Therefore, the presence of a free site in the coordination sphere or a site associated with a solvent, which is easily replaced by a substrate, is an important factor affecting the reactivity of metal complexes. For example, bis--allyl complexes of Ni are good precursors of catalytically active particles, since a complex with a solvent appears due to the easy reductive elimination of bis-allyl, the so-called. “Bare” nickel. The role of empty seats is illustrated by the following example:

Reactions of nucleophilic and electrophilic addition to - and-metal complexes

1. Reactions of organometallic compounds

As intermediates in catalytic reactions, there are both classical organometallic compounds having MC, M = C, and MC bonds, and nonclassical compounds in which the organic ligand is coordinated to the 2,  3,  4,  5, and 6 -types, or is an element of electron-deficient structures - bridging CH 3 and C 6 H 6 groups, nonclassical carbides (Rh 6 C (CO) 16, C (AuL) 5 +, C (AuL) 6 2+, etc.).

Among the specific mechanisms for classical -organometallic compounds, we note several mechanisms. Thus, 5 mechanisms of electrophilic substitution of a metal atom at the M-C bond have been established.

electrophilic substitution with nucleophilic assistance

AdE Attachment-Elimination

AdE (C) Attachment to the C atom of bsp 2 -hybridization

AdE (M) Addition oxidative to metal

Nucleophilic substitution at a carbon atom in demetallation reactions of organometallic compounds occurs as a redox process:

The participation of an oxidizing agent in such a stage is possible.

CuCl 2, p-benzoquinone, NO 3 - and other compounds can serve as such an oxidizing agent. Here are two more elementary stages characteristic of RMX:

hydrogenolysis of the M-C bond

and homolysis of the M-C bond

An important rule relating to all reactions of complex and organometallic compounds and associated with the principle of least motion is Tolman's 16-18 electron shell rule (Section 2).

The main substitution reaction in aqueous solutions - the exchange of water molecules (22) - has been studied for a large number of metal ions (Fig. 34). The exchange of water molecules of the coordination sphere of a metal ion with the bulk of water molecules, which is present as a solvent, for most metals proceeds very quickly, and therefore the rate of such a reaction was studied mainly by the relaxation method. The method consists in disrupting the equilibrium of the system, for example, by a sharp increase in temperature. Under new conditions (higher temperature), the system will no longer be in equilibrium. The rate of equilibration is then measured. If you can change the temperature of the solution within 10 -8 sec, then it is possible to measure the reaction rate, which requires for its completion a time interval greater than 10 -8 sec.

It is also possible to measure the rate of substitution of coordinated water molecules for various metal ions by the ligands SO 2-4, S 2 O 3 2-, EDTA, etc. (26). The speed of this reaction

depends on the concentration of the hydrated metal ion and does not depend on the concentration of the incoming ligand, which makes it possible to use the first-order equation (27) to describe the rate of these systems. In many cases, the rate of reaction (27) for a given metal ion does not depend on the nature of the incoming ligand (L), be it H2O molecules or SO 4 2-, S 2 O 3 2- or EDTA ions.

This observation, as well as the fact that the concentration of the incoming ligand is not included in the equation for the rate of this process, suggest that these reactions proceed by a mechanism in which the slow stage consists in breaking the bond between the metal ion and water. The resulting compound likely then quickly coordinates the nearby ligands.

In sect. 4 of this chapter, it was indicated that more highly charged hydrated metal ions, such as Al 3+ and Sc 3+, exchange water molecules more slowly than ions M 2+ and M +; this suggests that in the stage that determines the speed of the entire process, the breaking of bonds plays an important role. The conclusions obtained in these studies are not final, but they give reason to believe that S N 1 processes are of great importance in the substitution reactions of hydrated metal ions.

Probably the best studied complex compounds are cobalt (III) amines. Their stability, ease of preparation, and slow reactions with them make them especially convenient for kinetic studies. Since the studies of these complexes were carried out exclusively in aqueous solutions, one should first consider the reactions of these complexes with the solvent molecules - water. It was found that, in general, ammonia or amine molecules coordinated by the Co (III) ion are so slowly replaced by water molecules that substitution of other ligands rather than amines is usually considered.

The rate of reactions of the type (28) was studied and it was found that it is of the first order relative to the cobalt complex (X is one of the many possible anions).

Since in aqueous solutions the concentration of H 2 O is always approximately 55.5 M, then it is impossible to determine the effect of changes in the concentration of water molecules on the reaction rate. The rate equations (29) and (30) for an aqueous solution are not experimentally distinguishable, since k is simply equal to k "= k". Consequently, according to the reaction rate equation, it is impossible to say whether H 2 O will participate in the stage that determines the rate of the process. The answer to the question whether this reaction proceeds according to the S N 2 mechanism with the replacement of the X ion by the H 2 O molecule or by the S N 1 mechanism, which first involves dissociation followed by the addition of the H 2 O molecule, must be obtained using other experimental data.

The solution to this problem can be achieved by two types of experiments. Hydrolysis rate (substitution of one Cl ion - per water molecule) trance- + approximately 10 3 times the rate of hydrolysis 2+. An increase in the charge of the complex leads to an increase in the metal - ligand bonds, and, consequently, to inhibition of the breaking of these bonds. The attraction of the incoming ligands and the facilitation of the substitution reaction should also be taken into account. Since a decrease in the rate was found with an increase in the charge of the complex, in this case a dissociative process (S N 1) seems more likely.

Another method of proof is based on the study of hydrolysis of a series of complexes of similar trance- +. In these complexes, the ethylenediamine molecule is replaced by analogous diamines, in which the hydrogen atoms at the carbon atom are replaced by CH 3 groups. Complexes containing substituted diamines react faster than the ethylenediamine complex. The replacement of hydrogen atoms by CH 3 groups increases the volume of the ligand, which makes it difficult for the metal atom to be attacked by another ligand. These steric obstacles slow down the reaction by the S N 2 mechanism. The presence of bulky ligands near the metal atom promotes the dissociative process, since the removal of one of the ligands reduces their accumulation at the metal atom. The observed increase in the rate of hydrolysis of complexes with bulky ligands is good evidence of the reaction proceeding via the S N 1 mechanism.

So, as a result of numerous studies of the Co (II) acidoamine complexes, it turned out that the replacement of acidogroups by water molecules is by its nature a dissociative process. The cobalt atom-ligand bond is extended to a certain critical value before water molecules begin to enter the complex. In complexes with a charge of 2+ and higher, the cleavage of the cobalt - ligand bond is very difficult, and the entry of water molecules begins to play a more important role.

It was found that the replacement of the acid group (X -) in the cobalt (III) complex by a group other than the H2O molecule, (31) first goes through its substitution with a molecule

the solvent is water, followed by its replacement with a new group Y (32).

Thus, in many reactions with cobalt (III) complexes, the rate of reaction (31) is equal to the rate of hydrolysis (28). Only the hydroxyl ion differs from other reagents with respect to reactivity with Co (III) amines. It reacts very quickly with cobalt (III) ammine complexes (about 10 6 times faster than water) by the type of reaction basic hydrolysis (33).

It was found that this reaction is of the first order relative to the substitutional ligand OH - (34). The general second order of the reaction and the unusually fast course of the reaction suggest that the OH - ion is an extremely effective nucleophilic reagent with respect to the Co (III) complexes and that the reaction proceeds according to the S N 2 mechanism through the formation of an intermediate compound.

However, this property of OH - can also be explained by another mechanism [equations (35), (36)]. In reaction (35), complex 2+ behaves like an acid (according to Bronsted), giving complex +, which is amido- (containing) -compound - a base corresponding to acid 2+.

Then the reaction proceeds according to the S N 1 mechanism (36) with the formation of a five-coordinated intermediate compound, which then reacts with solvent molecules, which leads to the final reaction product (37). This reaction mechanism is consistent with the second-order reaction rate and corresponds to the S N 1 mechanism. Since the reaction in the stage determining the rate includes a base conjugated to the initial complex, an acid, this mechanism is given the designation S N 1CB.

Determining which of these mechanisms best explains experimental observations is very difficult. However, there is compelling evidence to support the S N 1CB hypothesis. The best arguments in favor of this mechanism are as follows: octahedral Co (III) complexes generally react according to the dissociative SN 1 mechanism, and there is no convincing reason why the OH - ion should cause the SN 2 process. It has been established that the hydroxyl ion is a weak nucleophilic reagent in reactions with Pt (II), and therefore its unusual reactivity with Co (III) seems to be unreasonable. Reactions with cobalt (III) compounds in non-aqueous media provide excellent evidence for the formation of five-coordination intermediates envisaged by the S N 1 CB mechanism.

The final proof is the fact that in the absence of N - H bonds in the Co (III) complex, it slowly reacts with OH - ions. This, of course, gives reason to believe that the acid-base properties of the complex are more important for the reaction rate than the nucleophilic properties of OH. " to exclude this or that possible mechanism, a rather subtle experiment must be carried out.

At present, substitution reactions of a large number of octahedral compounds have been investigated. If we consider their reaction mechanisms, then the most common is the dissociative process. This result is not unexpected, since the six ligands leave little room around the central atom for other groups to attach to it. Only a few examples are known when the formation of a seven-coordinated intermediate has been proven or the effect of an invading ligand has been detected. Therefore, the S N 2 mechanism cannot be completely rejected as a possible pathway for substitution reactions in octahedral complexes.

Conventionally, the chemical reactions of the complexes are subdivided into exchange, redox, isomerization, and coordinated ligands.

The primary dissociation of the complexes into the inner and outer spheres determines the course of the exchange reactions of the outer-sphere ions:

X m + mNaY = Y m + mNaX.

The components of the inner sphere of the complexes can also participate in exchange processes with the participation of both ligands and a complexing agent. To characterize the reactions of substitution of ligands or the central metal ion, the notation and terminology proposed by K. Ingold for reactions of organic compounds (Fig. 42), nucleophilic S N and electrophilic S E substitution:

Z + Y = z + X S N

Z + M "= z + M S E.

By the mechanism of the substitution reaction, they are divided (Fig. 43) into associative ( S N 1 and S E 1 ) and dissociative ( S N 2 and S E 2 ), differing in the transition state with increased and decreased coordination numbers.

The assignment of the reaction mechanism to associative or dissociative is a difficult experimentally achievable problem of identifying an intermediate with a reduced or increased coordination number. In this regard, the reaction mechanism is often judged on the basis of indirect data on the effect of the concentration of reagents on the reaction rate, changes in the geometric structure of the reaction product, etc.

To characterize the rate of ligand substitution reactions of complexes, the 1983 Nobel laureate G. Taube (Fig. 44) suggested using the terms “labile” and “inert” depending on the ligand substitution reaction time of less than or more than 1 minute. The terms labile or inert are characteristics of the kinetics of ligand substitution reactions and should not be confused with the thermodynamic characteristics of the stability or instability of complexes.

The lability or inertness of the complexes depends on the nature of the complexing ion and ligands. In accordance with the ligand field theory:

1. Octahedral complexes 3 d transition metals with a distribution of valence ( n -1) d electrons per sigma* (e g ) loosening MOs are labile.

4- (t 2g 6 e g 1) + H 2 O= 3- + CN -.

Moreover, the lower the value of the stabilization energy by the crystal field of the complex, the greater its lability.

2. Octahedral complexes 3 d free sigma transition metals* loosening e g orbitals and uniform distribution of valence ( n -1) d electrons in t 2 g orbitals (t 2 g 3, t 2 g 6) are inert.

[Co III (CN) 6] 3- (t 2 g 6 e g 0) + H 2 O =

[Cr III (CN) 6] 3- (t 2 g 3 e g 0) + H 2 O =

3. Plane-square and octahedral 4 d and 5 d transition metals having no electrons per sigma* Loosening MOs are inert.

2+ + H 2 O =

2+ + H 2 O =

The influence of the nature of ligands on the rate of ligand substitution reactions is considered within the framework of the model of “mutual influence of ligands”. A particular case of the model of the mutual influence of ligands is, formulated in 1926 by I.I. Chernyaev concept of trans-influence (Fig. 45) - "The ligand lability in the complex depends on the nature of the trans-located ligand" - and suggest a number of trans-effects of ligands: CO, CN -, C 2 H 4> PR 3, H -> CH 3 -, SC (NH 2) 2> C 6 H 5 -, NO 2 -, I -, SCN -> Br -, Cl -> py , NH 3, OH -, H 2 O.

The concept of trans influence allowed to substantiate the rules of thumb:

1. Peyron's Rule- under the action of ammonia or amines on tetraloplatinate ( II ) potassium is always obtained dichlodiamine platinum of the cis-configuration:

2 - + 2NH 3 = cis - + 2Cl -.

Since the reaction proceeds in two stages and the chloride ligand has a large trans-effect, the replacement of the second chloride ligand with ammonia occurs with the formation of cis- [ Pt (NH 3) 2 Cl 2]:

2- + NH 3 = -

NH 3 = cis -.

2. Jergensen's rule - under the action of hydrochloric acid on platinum tetrammine chloride ( II ) or similar compounds, trans-dichlorodiammineplatinum is obtained:

[Pt (NH 3) 4] 2+ + 2 HCl = trans- [Pt (NH 3) 2 Cl 2] + 2 NH 4 Cl.

In accordance with a number of trans-effect of ligands, the replacement of the second ammonia molecule with a chloride ligand leads to the formation of trans- [ Pt (NH 3) 2 Cl 2].

3. Kurnakov's thiourea reaction - various reaction products of thiomo-chevin with geometric isomers of trans- [ Pt (NH 3) 2 Cl 2] and cis- [Pt (NH 3) 2 Cl 2]:

cis - + 4Thio = 2+ + 2Cl - + 2NH 3.

The different nature of the reaction products is associated with the high trans-effect of thiourea. The first stage of the reactions is the substitution of thiourea chloride ligands with the formation of trans- and cis- [ Pt (NH 3) 2 (Thio) 2] 2+:

trans- [Pt (NH 3) 2 Cl 2] + 2 Thio = trans- [Pt (NH 3) 2 (Thio) 2] 2+

cis - + 2Thio = cis - 2+.

In cis- [Pt (NH 3) 2 (Thio ) 2] 2+ two ammonia molecules in the trans position to thiourea undergo further substitution, which leads to the formation 2+ :

cis - 2+ + 2Thio = 2+ + 2NH 3.

In trans- [Pt (NH 3) 2 (Thio ) 2] 2+ two ammonia molecules with a small trans-influence are located in the trans position to each other and therefore are not replaced by thiourea.

The patterns of trans-influence were discovered by I.I. Chernyaev in the study of ligand substitution reactions in flat-square platinum complexes ( II ). Later, it was shown that the trans effect of ligands is also manifested in complexes of other metals ( Pt (IV), Pd (II), Co (III), Cr (III), Rh (III), Ir (III )) and other geometric structures. True, the series of the trans effect of ligands for different metals are somewhat different.

It should be noted that trans influence is kinetic effect- the more trans-influence a given ligand has, the more rapidly another ligand, which is in the trans position with respect to it, is replaced.

Along with the kinetic trance effect, in the middle XX century A.A. Grinberg and Yu.N. Kukushkin established the dependence of the trans-influence of the ligand L from the ligand in the cis position to L ... Thus, the study of the rate of the substitution reaction Cl - ammonia in platinum complexes ( II):

[PtCl 4] 2- + NH 3 = [PtNH 3 Cl 3] - + Cl - K = 0.42. 10 4 L / mol. with

[PtNH 3 Cl 3] - + NH 3 = cis- [Pt (NH 3) 2 Cl 2] + Cl - K = 1.14. 10 4 L / mol. with

trans- [Pt (NH 3) 2 Cl 2] + NH 3 = [Pt (NH 3) 3 Cl] + + Cl - K = 2.90. 10 4 L / mol. with

showed that the presence of one and two ammonia molecules in the cis-position to the substituted chloride ligand leads to a sequential increase in the reaction rate. This kinetic effect is called cis influence... Currently, both kinetic effects of the influence of the nature of ligands on the rate of ligand substitution reactions (trans- and cis-influence) are combined in a general concept mutual influence of ligands.

The theoretical substantiation of the effect of the mutual influence of ligands is closely related to the development of ideas about the chemical bond in complex compounds. In the 30s XX century A.A. Grinberg and B.V. Nekrasov considered the trans influence within the framework of the polarization model:

1. The trans effect is typical for complexes, the central metal ion of which has a high polarizability.

2. The trans activity of ligands is determined by the value of the mutual polarization energy of the ligand and the metal ion. For a given metal ion, the trans effect of the ligand is determined by its polarizability and distance from the central ion.

The polarization model is consistent with experimental data for complexes with simple anionic ligands, for example, halide ions.

In 1943 A.A. Greenberg suggested that the trans activity of the ligands is related to their reducing properties. The shift of the electron density from the trans-active ligand to the metal decreases the effective charge of the metal ion, which leads to a weakening chemical bond with a trans-located ligand.

The development of ideas about the trans-effect is associated with the high trans-activity of ligands based on unsaturated organic molecules like ethylene in [ Pt (C 2 H 4) Cl 3 ] -. According to Chatt and Orgel (Fig. 46), this is due topi-the dative interaction of such ligands with the metal; and the associative mechanism of substitution reactions of trans-located ligands. Coordination to the metal ion of the attacking ligand Z leads to the formation of a five-coordinated trigonal-bipyramid intermediate with subsequent rapid elimination of the leaving ligand X.pi-dative ligand-metal ligand interaction Y , which decreases the electron density of the metal and decreases the activation energy of the transition state with the subsequent rapid replacement of the ligand X.

As well as p acceptor (C 2 H 4, CN -, CO ...) ligands that form a dative ligand-metal chemical bond, have a high trans-influence andsdonor ligands: H -, CH 3 -, C 2 H 5 - ... The trans effect of such ligands is determined by the donor-acceptor interaction of the ligand X with the metal, which lowers its electron density and weakens the bond of the metal with the leaving ligand Y.

Thus, the position of ligands in the series of trans activities is determined by the joint action of sigma donor and pi-properties of ligands - sigma- donor and pi-the acceptor properties of the ligand enhance its trans effect, whilepi-donor - weaken. Which of these components of the ligand-metal interaction prevails in the trans-effect is judged on the basis of quantum-chemical calculations of the electronic structure of the transition state of the reaction.

General chemistry: textbook / A. V. Zholnin; ed. V. A. Popkova, A. V. Zholnina. - 2012 .-- 400 p .: ill.

Chapter 7. COMPLEX CONNECTIONS

Chapter 7. COMPLEX CONNECTIONS

Complexing elements are the organizers of life.

K. B. Yatsimirsky

Complex compounds are the most extensive and diverse class of compounds. Living organisms contain complex compounds of biogenic metals with proteins, amino acids, porphyrins, nucleic acids, carbohydrates, macrocyclic compounds. The most important vital processes occur with the participation of complex compounds. Some of them (hemoglobin, chlorophyll, hemocyanin, vitamin B 12, etc.) play a significant role in biochemical processes. Many medicines contain metal complexes. For example, insulin (zinc complex), vitamin B 12 (cobalt complex), platinum (platinum complex), etc.

7.1. COORDINATION THEORY OF A. WERNER

The structure of complex compounds

During the interaction of particles, mutual coordination of particles is observed, which can be defined as a complexation process. For example, the process of ion hydration ends with the formation of aqua complexes. Complexation reactions are accompanied by the transfer of electron pairs and lead to the formation or destruction of higher-order compounds, the so-called complex (coordination) compounds. A feature of complex compounds is the presence of a coordination bond in them, which has arisen by the donor-acceptor mechanism:

Complex compounds are compounds that exist both in the crystalline state and in solution, a feature

which is the presence of a central atom surrounded by ligands. Complex compounds can be considered as complex compounds of a higher order, consisting of simple molecules capable of independent existence in solution.

According to Werner's coordination theory, a complex compound is distinguished internal and outer sphere. The central atom with the surrounding ligands form the inner sphere of the complex. It is usually enclosed in square brackets. Everything else in the complex compound makes up the outer sphere and is written outside square brackets. A certain number of ligands is placed around the central atom, which is determined by coordination number(kh). The number of coordinated ligands is most often 6 or 4. The ligand occupies a coordination position near the central atom. Coordination changes the properties of both the ligands and the central atom. Often, coordinated ligands cannot be detected using chemical reactions that are characteristic of them in a free state. The more tightly bound particles of the inner sphere are called complex (complex ion). Attraction forces act between the central atom and the ligands (a covalent bond is formed according to the exchange and (or) donor-acceptor mechanism), between the ligands - repulsive forces. If the charge of the inner sphere is 0, then the outer coordination sphere is absent.

Central atom (complexing agent)- an atom or ion that occupies a central position in a complex compound. The role of the complexing agent is most often performed by particles with free orbitals and a sufficiently large positive nuclear charge, and therefore, can be electron acceptors. These are cations of transition elements. The strongest complexing agents are elements of groups IB and VIIIB. Rarely as a complex

The authors are neutral atoms of d-elements and atoms of non-metals in various oxidation states -. The number of free atomic orbitals provided by the complexing agent determines its coordination number. The value of the coordination number depends on many factors, but usually it is equal to twice the charge of the complexing ion:

Ligands- ions or molecules that are directly associated with the complexing agent and are donors of electron pairs. These electron-rich systems, which have free and mobile electron pairs, can be electron donors, for example:

Compounds of p-elements exhibit complexing properties and act as ligands in a complex compound. Ligands can be atoms and molecules (protein, amino acids, nucleic acids, carbohydrates). According to the number of bonds formed by ligands with a complexing agent, ligands are divided into mono-, di-, and polydentate ligands. The above ligands (molecules and anions) are monodentate, since they are donors of one electron pair. Bidentate ligands include molecules or ions containing two functional groups capable of donating two electron pairs:

Polydentate ligands include the 6-dentate ligand of ethylenediaminetetraacetic acid:

The number of sites occupied by each ligand in the inner sphere of the complex compound is called coordination capacity (denticity) of the ligand. It is determined by the number of electron pairs of the ligand, which are involved in the formation of a coordination bond with the central atom.

In addition to complex compounds, coordination chemistry encompasses double salts, crystalline hydrates, which decompose in an aqueous solution into their constituent parts, which in the solid state in many cases are constructed similarly to complex ones, but are unstable.

The most stable and diverse complexes in terms of composition and functions they perform form d-elements. Complex compounds of transition elements: iron, manganese, titanium, cobalt, copper, zinc and molybdenum are of particular importance. Biogenic s -elements (Na, K, Mg, Ca) form complex compounds only with ligands of a certain cyclic structure, also acting as a complexing agent. Main part R-elements (N, P, S, O) is an active active part of complexing particles (ligands), including bioligands. This is their biological significance.

Consequently, the ability to form a complex is a common property of the chemical elements of the periodic system, this ability decreases in the following order: f> d> p> s.

7.2. DETERMINATION OF THE CHARGE OF THE MAIN PARTICLES OF THE COMPLEX JOINT

The charge of the inner sphere of the complex compound is the algebraic sum of the charges of the particles that form it. For example, the magnitude and sign of the charge of a complex are determined as follows. The charge of the aluminum ion is +3, the total charge of the six hydroxide ions is -6. Therefore, the charge of the complex is (+3) + (-6) = -3 and the formula of the complex is 3-. The charge of a complex ion is numerically equal to the total charge of the outer sphere and opposite in sign. For example, the charge of the outer sphere K 3 is +3. Therefore, the charge of the complex ion is -3. The charge of the complexing agent is equal in magnitude and opposite in sign to the algebraic sum of the charges of all other particles of the complex compound. Hence, in K 3 the charge of the iron ion is +3, since the total charge of all other particles of the complex compound is (+3) + (-6) = -3.

7.3. NOMENCLATURE OF COMPLEX JOINTS

The basics of nomenclature are developed in the classic works of Werner. In accordance with them, in a complex compound, the cation is first called, and then the anion. If the compound is of a non-electrolyte type, then it is called in one word. The name of a complex ion is written in one word.

The neutral ligand is called the same as the molecule, and is added to the ligand anions at the end of the "o". For a coordinated water molecule, the designation "aqua-" is used. To designate the number of identical ligands in the inner sphere of the complex, the Greek numerals di-, tri-, tetra-, penta-, hexa-, etc. are used as a prefix before the name of the ligands. The monone prefix is ​​used. Ligands are listed alphabetically. The name of the ligand is considered as a whole. The name of the ligand is followed by the name of the central atom with the indication of the oxidation state, which is denoted by Roman numerals in parentheses. The word ammin (with two "m") is written in relation to ammonia. For all other amines, only one "m" is used.

C1 3 - hexamminecobalt (III) chloride.

C1 3 - aquapentammincobalt (III) chloride.

Cl 2 - pentamethylammine chlorocobalt (III) chloride.

Diammindibromoplatin (II).

If the complex ion is an anion, then its Latin name has the ending "am".

(NH 4) 2 - ammonium tetrachloropalladate (II).

K - potassium pentabromoammineplatinate (IV).

K 2 - potassium tetrarodanocobaltate (II).

The name of a complex ligand is usually enclosed in parentheses.

NO 3 - dichloro-di- (ethylenediamine) cobalt (III) nitrate.

Br - bromo-tris- (triphenylphosphine) platinum (II) bromide.

In cases where the ligand binds two central ions, the Greek letter is used before its nameμ.

Such ligands are called bridge and listed last.

7.4. CHEMICAL BONDING AND STRUCTURE OF COMPLEX COMPOUNDS

Donor-acceptor interactions between the ligand and the central atom play an important role in the formation of complex compounds. The donor of an electron pair is usually a ligand. The acceptor is the central atom, which has free orbitals. This bond is strong and does not break when the complex dissolves (neionogenic), and it is called coordination.

Along with o-bonds, π-bonds are formed by the donor-acceptor mechanism. In this case, a metal ion serves as a donor, donating its paired d-electrons to the ligand, which has energetically favorable vacant orbitals. Such connections are called dative. They are formed:

a) due to the overlapping of the vacant p-orbitals of the metal with the d-orbital of the metal, on which there are electrons that have not entered into a σ-bond;

b) when the vacant d-orbitals of the ligand overlap with filled d-orbitals of the metal.

A measure of its strength is the degree of overlap of the orbitals of the ligand and the central atom. The orientation of the bonds of the central atom determines the geometry of the complex. To explain the directionality of bonds, the concept of hybridization of atomic orbitals of the central atom is used. Hybrid orbitals of the central atom are the result of mixing of unequal atomic orbitals, as a result, the shape and energy of the orbitals mutually change, and new orbitals of the same shape and energy are formed. The number of hybrid orbitals is always equal to the number of original ones. Hybrid clouds are located in an atom at the maximum distance from each other (Table 7.1).

Table 7.1. Types of hybridization of atomic orbitals of a complexing agent and the geometry of some complex compounds

The spatial structure of the complex is determined by the type of hybridization of the valence orbitals and the number of lone electron pairs contained in its valence energy level.

The efficiency of the donor-acceptor interaction of the ligand and the complexing agent, and, consequently, the strength of the bond between them (the stability of the complex) is determined by their polarizability, i.e. the ability to transform their electronic shells under external influence. On this basis, reagents are subdivided into "Tough" or low polarizable, and "Soft" - readily polarizable. The polarity of an atom, molecule or ion depends on their size and the number of electronic layers. The smaller the radius and electrons of a particle, the less polarized it is. The smaller the radius and fewer electrons of a particle, the worse it is polarized.

Hard acids form strong (hard) complexes with electronegative atoms O, N, F of ligands (hard bases), and soft acids form strong (soft) complexes with donor atoms P, S and I of ligands with low electronegativity and high polarizability. We see a manifestation here general principle"Like with like".

Sodium and potassium ions, due to their rigidity, practically do not form stable complexes with biosubstrates and in physiological media are in the form of aqua complexes. Ions Ca 2 + and Mg 2 + form fairly stable complexes with proteins and therefore in physiological media are both in the ionic and in the bound state.

Ions of d-elements form strong complexes with biosubstrates (proteins). Soft acids Cd, Pb, Hg are highly toxic. They form strong complexes with proteins containing R-SH sulfhydryl groups:

Cyanide ion is toxic. The soft ligand actively interacts with d-metals in complexes with biosubstrates, activating the latter.

7.5. DISSOCIATION OF COMPLEX JOINTS. SUSTAINABILITY OF COMPLEXES. LABLE AND INERT COMPLEXES

When complex compounds are dissolved in water, they usually decompose into ions of the outer and inner spheres, like strong electrolytes, since these ions are ionically bound, mainly by electrostatic forces. This is assessed as the primary dissociation of complex compounds.

Secondary dissociation of a complex compound is the disintegration of the inner sphere into its constituent components. This process proceeds according to the type of weak electrolytes, since the particles of the inner sphere are connected nonionically (by covalent bonds). Dissociation is of a stepwise nature:

To qualitatively characterize the stability of the inner sphere of a complex compound, an equilibrium constant is used that describes its complete dissociation, called complex instability constant(Book). For a complex anion, the expression for the instability constant has the form:

The smaller the Kn value, the more stable the inner sphere of the complex compound is, i.e. the less it dissociates in aqueous solution. V recent times instead of Kn, the value of the stability constant (Ku) is used - the reciprocal of Kn. The higher the Ku value, the more stable the complex.

Stability constants allow predicting the direction of ligand exchange processes.

In an aqueous solution, a metal ion exists in the form of aquacomplexes: 2 + - hexaaqua iron (II), 2 + - tetraaqua copper (II). When writing formulas for hydrated ions, we do not indicate the coordinated water molecules of the hydration shell, but mean it. The formation of a complex between a metal ion and a ligand is considered as a reaction of the substitution of a water molecule in the inner coordination sphere by this ligand.

Ligand exchange reactions proceed according to the S N -type reaction mechanism. For example:

The values ​​of the stability constants given in Table 7.2 indicate that due to the complexation process, strong binding of ions in aqueous solutions occurs, which indicates the efficiency of using this type of reactions for binding ions, especially with polydentate ligands.

Table 7.2. Stability of zirconium complexes

Unlike ion exchange reactions, the formation of complex compounds is often not a quasi-instantaneous process. For example, when iron (III) interacts with nitrilotrimethylenephosphonic acid, equilibrium is established after 4 days. For the kinetic characteristics of the complexes, the concepts are used - labile(quickly reacting) and inert(slowly reacting). Labile complexes, at the suggestion of G. Taube, are those that fully exchange ligands for 1 min at room temperature and a solution concentration of 0.1 M. It is necessary to clearly distinguish between thermodynamic concepts [strong (stable) / fragile (unstable)] and kinetic [ inert and labile] complexes.

In labile complexes, ligand substitution occurs quickly and equilibrium is quickly established. In inert complexes, the ligand substitution proceeds slowly.

Thus, the inert complex 2 + in an acidic medium is thermodynamically unstable: the instability constant is 10 -6, and the labile complex 2- is very stable: the stability constant is 10 -30. Taube associates the lability of the complexes with the electronic structure of the central atom. The inertness of the complexes is characteristic mainly of ions with an unfinished d-shell. Complexes Co, Cr are inert. Cyanide complexes of many cations with an external s 2 p 6 level are labile.

7.6. CHEMICAL PROPERTIES OF THE COMPLEXES

Complexation processes affect practically the properties of all particles that form the complex. The higher the strength of the bonds between the ligand and the complexing agent, the less the properties of the central atom and ligands are manifested in the solution and the more pronounced are the features of the complex.

Complex compounds exhibit chemical and biological activity as a result of coordination unsaturation of the central atom (there are free orbitals) and the presence of free electron pairs of ligands. In this case, the complex has electrophilic and nucleophilic properties that differ from those of the central atom and ligands.

It is necessary to take into account the effect on the chemical and biological activity of the structure of the hydration shell of the complex. The process of education

the formation of complexes affects the acid-base properties of the complex compound. The formation of complex acids is accompanied by an increase in the strength of the acid or base, respectively. So, when complex acids are formed from simple ones, the binding energy with H + ions decreases and the acid strength increases accordingly. If there is an OH - ion in the outer sphere, then the bond between the complex cation and the hydroxide ion of the outer sphere decreases, and the basic properties of the complex increase. For example, copper hydroxide Cu (OH) 2 is a weak, hardly soluble base. When ammonia acts on it, copper ammonia (OH) 2 is formed. The charge density of 2 +, compared to Cu 2 +, decreases, the bond with OH - ions is weakened, and (OH) 2 behaves like a strong base. The acid-base properties of the ligands associated with the complexing agent are usually more pronounced than their acid-base properties in the free state. For example, hemoglobin (Hb) or oxyhemoglobin (HbO 2) exhibit acidic properties due to free carboxyl groups of globin protein, which is a ligand of ННb ↔ Н + + Hb -. At the same time, the hemoglobin anion, due to the amino groups of the globin protein, exhibits basic properties and therefore binds the acidic oxide CO 2 to form the carbaminoglobin anion (НbСО 2 -): СО 2 + Hb - ↔ НbСО 2 -.

The complexes exhibit redox properties due to redox transformations of the complex-former, which forms stable oxidation states. The complexation process strongly affects the values ​​of the reduction potentials of d-elements. If the reduced form of cations forms a more stable complex with a given ligand than its oxidized form, then the value of the potential increases. A decrease in the potential value occurs when an oxidized form forms a more stable complex. For example, under the action of oxidants: nitrites, nitrates, NO 2, H 2 O 2, hemoglobin is converted into methemoglobin as a result of oxidation of the central atom.

The sixth orbital is used in the formation of oxyhemoglobin. The same orbital is involved in the formation of a bond with carbon monoxide. As a result, a macrocyclic complex with iron is formed - carboxyhemoglobin. This complex is 200 times more stable than the iron-oxygen complex in heme.

Rice. 7.1. Chemical transformations of hemoglobin in the human body. Scheme from the book: Slesarev V.I. Fundamentals of Chemistry of the Living, 2000

The formation of complex ions affects the catalytic activity of the ions of the complexing agents. In some cases, the activity increases. This is due to the formation in solution of large structural systems capable of participating in the creation of intermediate products and a decrease in the activation energy of the reaction. For example, if you add Cu 2+ or NH 3 to H 2 O 2, the decomposition process is not accelerated. In the presence of the complex 2 +, which is formed in an alkaline environment, the decomposition of hydrogen peroxide is accelerated 40 million times.

So, on hemoglobin, you can consider the properties of complex compounds: acid-base, complexation and redox.

7.7. CLASSIFICATION OF COMPLEX JOINTS

There are several systems for the classification of complex compounds, which are based on different principles.

1.According to the belonging of a complex compound to a certain class of compounds:

Complex acids H 2;

Complex bases OH;

Complex salts K 4.

(2) By the nature of the ligand: aqua complexes, ammoniaates, acidocomplexes (anions of various acids, K 4; hydroxo complexes (hydroxyl groups, K 3 act as ligands); complexes with macrocyclic ligands, inside which are located central atom.

3. According to the sign of the charge of the complex: cationic - complex cation in the complex compound Cl 3; anionic - a complex anion in a complex compound K; neutral - the charge of the complex is 0. The complex compound of the external sphere does not have, for example. This is an anticancer drug formula.

4.According to the internal structure of the complex:

a) depending on the number of atoms of the complexing agent: mononuclear- the complex particle contains one atom of the complexing agent, for example, Cl 3; multicore- in the composition of the complex particle there are several complex-forming atoms - an iron-protein complex:

b) depending on the number of types of ligands, complexes are distinguished: homogeneous (single-ligand), containing one type of ligand, for example 2+, and dissimilar (mixed ligand)- two kinds of ligands or more, for example Pt (NH 3) 2 Cl 2. The complex includes ligands NH 3 and Cl -. For complex compounds containing different ligands in the inner sphere, geometric isomerism is characteristic, when, with the same composition of the inner sphere, the ligands in it are arranged differently relative to each other.

Geometric isomers of complex compounds differ not only in physical and chemical properties, but also in biological activity. The cis isomer of Pt (NH 3) 2 Cl 2 has a pronounced antitumor activity, while the trans isomer does not;

c) depending on the denticity of the ligands forming mononuclear complexes, groups can be distinguished:

Mononuclear complexes with monodentate ligands, for example 3+;

Mononuclear complexes with polydentate ligands. Complex compounds with polydentate ligands are called chelating compounds;

d) cyclic and acyclic forms of complex compounds.

7.8. CHELATE COMPLEXES. COMPLEXONS. COMPLEXES

Cyclic structures that form as a result of the attachment of a metal ion to two or more donor atoms belonging to the same molecule of the chelating agent are called chelated compounds. For example, copper glycinate:

In them, the complexing agent, as it were, leads into the ligand, is engulfed in bonds, like claws, therefore, other things being equal, they have a higher stability than compounds that do not contain cycles. The most stable cycles are those with five or six links. This rule was first formulated by L.A. Chugaev. Difference

the stability of the chelate complex and the stability of its non-cyclic analogue are called chelation effect.

Polydentate ligands, which contain 2 types of groups, act as a chelating agent:

1) groups capable of forming covalent polar bonds due to exchange reactions (proton donors, acceptors of electron pairs) -CH 2 COOH, -CH 2 PO (OH) 2, -CH 2 SO 2 OH, - acid groups (centers);

2) electron pair donor groups: ≡N,> NH,> C = O, -S-, -OH, - the main groups (centers).

If such ligands saturate the inner coordination sphere of the complex and completely neutralize the charge of the metal ion, then the compounds are called inside the complex. For example, copper glycinate. There is no outer sphere in this complex.

A large group of organic substances containing basic and acidic centers in the molecule is called complexons. These are polybasic acids. Chelated compounds formed by complexones when interacting with metal ions are called complexonates, for example a magnesium complexonate with ethylenediaminetetraacetic acid:

In aqueous solution, the complex exists in anionic form.

Complexones and complexonates are a simple model of more complex compounds of living organisms: amino acids, polypeptides, proteins, nucleic acids, enzymes, vitamins, and many other endogenous compounds.

Currently, a huge range of synthetic chelators with various functional groups is being produced. The formulas of the main complexons are presented below:

Complexones, under certain conditions, can provide lone electron pairs (several) for the formation of a coordination bond with a metal ion (s-, p- or d-element). As a result, stable compounds of the chelate type with 4-, 5-, 6- or 8-membered rings are formed. This reaction takes place over a wide range of pH. Depending on the pH, the nature of the complexing agent, its ratio with the ligand, complexonates of various strength and solubility are formed. The chemistry of the formation of complexonates can be represented by the equations for the sodium salt of EDTA (Na 2 H 2 Y), which dissociates in an aqueous solution: Na 2 H 2 Y → 2Na + + H 2 Y 2-, and the H 2 Y 2- ion interacts with ions metals, regardless of the oxidation state of the metal cation, most often one metal ion (1: 1) interacts with one complexone molecule. The reaction proceeds quantitatively (Кр> 10 9).

Complexones and complexonates exhibit amphoteric properties in a wide pH range, the ability to participate in oxidation-reduction reactions, complexation, form compounds with various properties depending on the oxidation state of the metal, its coordination saturation, and have electrophilic and nucleophilic properties. All this determines the ability to bind a huge number of particles, which allows solving large and varied problems with a small amount of reagent.

Another indisputable advantage of chelating agents and chelating agents is low toxicity and the ability to transform toxic particles

in low toxic or even biologically active. Decomposition products of complexonates do not accumulate in the body and are harmless. The third feature of complexonates is the possibility of using them as a source of trace elements.

The increased digestibility is due to the fact that the microelement is introduced in a biologically active form and has a high membrane permeability.

7.9. PHOSPHOROSE-CONTAINING COMPLEXONATES OF METALS - EFFECTIVE FORM OF CONVERSION OF MICRO- AND MACROELEMENTS INTO A BIOLOGICALLY ACTIVE STATE AND A MODEL FOR STUDYING THE BIOLOGICAL EFFECTS OF CHEMICAL ELEMENTS

Concept biological activity covers a wide range of phenomena. From the point of view of chemical action, biologically active substances (BAS) are usually understood as substances that can act on biological systems, regulating their vital activity.

The ability to such an effect is interpreted as the ability to manifest biological activity. Regulation can manifest itself in the effects of stimulation, oppression, development of certain effects. An extreme manifestation of biological activity is biocidal action, when, as a result of the effect of a biocide substance on the body, the latter dies. At lower concentrations, in most cases, biocides have a stimulating, rather than lethal, effect on living organisms.

Currently known big number such substances. Nevertheless, in many cases, the use of known biologically active substances is used insufficiently, often with an efficiency far from maximum, and the use often leads to side effects that can be eliminated by introducing modifiers into the biologically active substances.

Phosphorus-containing complexonates form compounds with various properties depending on the nature, oxidation state of the metal, coordination saturation, composition and structure of the hydration shell. All this determines the polyfunctionality of complex-nates, their unique ability of substoichiometric action,

the effect of the common ion and provides wide application in medicine, biology, ecology and in various sectors of the national economy.

When the complexone is coordinated by the metal ion, a redistribution of the electron density occurs. Due to the participation of the lone electron pair in the donor-acceptor interaction, the electron density of the ligand (complexone) is shifted to the central atom. Lowering the relatively negative charge on the ligand reduces the Coulomb repulsion of the reactants. Therefore, the coordinated ligand becomes more accessible for attack by a nucleophilic reagent with an excess of electron density at the reaction center. The shift of the electron density from the chelator to the metal ion leads to a relative increase in the positive charge of the carbon atom, and, consequently, to the facilitation of its attack by the nucleophilic reagent, the hydroxyl ion. The hydroxylated complex among the enzymes that catalyze metabolic processes in biological systems occupies one of the central places in the mechanism of enzymatic action and detoxification of the body. As a result of the multipoint interaction of the enzyme with the substrate, orientation occurs, which ensures the convergence of the active groups in the active center and the transfer of the reaction to the intramolecular mode, before the start of the reaction and the formation of a transition state, which ensures the enzymatic function of PCM. Conformational changes can occur in enzyme molecules. Coordination creates additional conditions for the redox interaction between the central ion and the ligand, since a direct bond is established between the oxidizing agent and the reducing agent, which ensures the transition of electrons. Transition metal complexes of PCM can be characterized by electron transitions type L-M, M-L, M-L-M, in which the orbitals of both the metal (M) and the ligands (L) participate, which, respectively, are linked in the complex by donor-acceptor bonds. Complexones can serve as a bridge over which electrons of multinuclear complexes oscillate between the central atoms of one or different elements in different oxidation states (complexes of the transfer of electrons and protons). Complexones determine the reducing properties of metal complexonates, which allows them to exhibit high antioxidant, adaptogenic properties, and homeostatic functions.

So, chelators convert microelements into a biologically active form accessible to the body. They form stable,

more coordination-saturated particles, incapable of destroying biocomplexes, and, consequently, low-toxic forms. Complexonates act favorably in violation of microelement homeostasis of the body. Ions of transition elements in complexonate form act in the body as a factor that determines the high sensitivity of cells to microelements by their participation in creating a high concentration gradient, membrane potential. Transition metal complexonates FKM have bioregulatory properties.

The presence of acidic and basic centers in PCM provides amphoteric properties and their participation in maintaining acid-base equilibrium (isohydric state).

With an increase in the number of phosphonic groups in the complexone, the composition and conditions for the formation of soluble and poorly soluble complexes change. An increase in the number of phosphonic groups favors the formation of poorly soluble complexes in a wider pH range and shifts the region of their existence to the acidic region. The decomposition of the complexes occurs at a pH of more than 9.

The study of the processes of complexation with complexones made it possible to develop methods for the synthesis of bioregulators:

Prolonged-acting growth stimulants in colloidal-chemical form are polynuclear homo- and heterocomplex compounds of titanium and iron;

Growth stimulants in water-soluble form. These are mixed-ligand titanium chelating agents based on chelating agents and an inorganic ligand;

Growth inhibitors - phosphorus-containing s-element complexonates.

The biological effect of the synthesized drugs on growth and development was studied in a chronic experiment on plants, animals and humans.

Bioregulation- This is a new scientific direction that allows you to regulate the direction and intensity of biochemical processes, which can be widely used in medicine, animal husbandry and plant growing. It is associated with the development of recovery methods physiological function organism in order to prevent and treat diseases and age-related pathologies. Complexones and complex compounds based on them can be classified as promising biologically active compounds. The study of their biological action in a chronic experiment showed that chemistry has put into the hands of physicians,

livestock breeders, agronomists and biologists is a promising new tool that allows you to actively influence a living cell, regulate nutritional conditions, growth and development of living organisms.

The study of the toxicity of the used chelating agents and chelating agents showed a complete absence of the effect of the drugs on the blood-forming organs, blood pressure, excitability, respiratory rate: no change in liver function was noted, no toxicological effect on the morphology of tissues and organs was revealed. The potassium salt of OEDP does not have toxicity at a dose 5-10 times higher than the therapeutic dose (10-20 mg / kg) when studied for 181 days. Therefore, chelators are low-toxic compounds. They are used as medicines to combat viral diseases, poisoning with heavy metals and radioactive elements, impaired calcium metabolism, endemic diseases and imbalance of a trace element in the body. Phosphorus-containing complexes and complexonates do not undergo photolysis.

The progressive pollution of the environment with heavy metals - products of human economic activity - is a permanent environmental factor. They can build up in the body. Excess and deficiency of them cause intoxication of the body.

Metal chelating agents retain the chelating effect of the ligand (chelating agent) in the body and are indispensable for maintaining metal-ligand homeostasis. Incorporated heavy metals are neutralized to a certain extent in the body, and low resorption capacity prevents the transfer of metals along trophic chains, as a result of which leads to a certain "biominization" of their toxic effect, which is especially important for the Ural region. For example, free lead ion belongs to thiol poisons, and strong lead complexonate with ethylenediaminetetraacetic acid has low toxicity. Therefore, the detoxification of plants and animals consists in the use of metal complexonates. It is based on two thermodynamic principles: their ability to form strong bonds with toxic particles, converting them into poorly soluble or stable compounds in aqueous solution; their inability to destroy endogenous biocomplexes. In this regard, we believe that complex therapy of plants and animals is an important area of ​​combating eco-poisoning and obtaining environmentally friendly products.

The study of the effect of processing plants with complexonates of various metals with intensive cultivation technology

potatoes for the trace element composition of potato tubers. Tuber samples contained 105-116 mg / kg iron, 16-20 mg / kg manganese, 13-18 mg / kg copper and 11-15 mg / kg zinc. The ratio and content of trace elements are typical for plant tissues. Tubers grown with and without the use of metal complexonates have practically the same elemental composition. The use of chelates does not create conditions for accumulation heavy metals in tubers. Complexonates, to a lesser extent than metal ions, are sorbed by the soil, are resistant to its microbiological effect, which allows them to remain in the soil solution for a long time. The aftereffect is 3-4 years. They go well with various pesticides. The metal in the complex has a lower toxicity. Phosphorus-containing metal complexonates do not irritate the mucous membrane of the eyes and do not damage the skin. No sensitizing properties have been identified, the cumulative properties of titanium complexonates are not expressed, and in some they are very weak. The cumulation coefficient is 0.9-3.0, which indicates a low potential danger of chronic drug poisoning.

The phosphorus-containing complexes are based on the phosphorus-carbon bond (CP), which is also found in biological systems. It is part of phosphonolipids, phosphonoglycans and phosphoproteins of cell membranes. Lipids containing aminophosphonic compounds are resistant to enzymatic hydrolysis, provide stability and, consequently, the normal functioning of the outer cell membranes. Synthetic analogs of pyrophosphates - diphos-phonates (Р-С-Р) or (Р-С-С-Р) in large doses disrupt calcium metabolism, and in small doses they normalize it. Diphosphonates are effective in hyperlipemia and are pharmacologically promising.

Diphosphonates containing Р-С-Р bonds are structural elements of biosystems. They are biologically effective and are analogous to pyrophosphates. It has been shown that diphosphonates are effective means of treating various diseases. Diphosphonates are active inhibitors of bone mineralization and resorption. Complexones convert microelements into a biologically active form accessible to the body, form stable, more coordination-saturated particles that are incapable of destroying biocomplexes, and therefore, low-toxic forms. They determine the high sensitivity of cells to trace elements, participating in the formation of a high concentration gradient. Able to participate in the formation of multinuclear titanium compounds heteronuclear

of a different type - electron and proton transfer complexes, to participate in the bioregulation of metabolic processes, the body's resistance, the ability to form bonds with toxic particles, turning them into poorly soluble or soluble, stable, non-destructive endogenous complexes. Therefore, their use for detoxification, elimination from the body, obtaining environmentally friendly products (complex therapy), as well as in industry for the regeneration and disposal of industrial waste of inorganic acids and transition metal salts is very promising.

7.10. LIGANDEXCHANGE AND METAL EXCHANGE

BALANCE. CHELATOTHERAPY

If the system contains several ligands with one metal ion or several metal ions with one ligand capable of forming complex compounds, then competing processes are observed: in the first case, ligand-exchange equilibrium is competition between ligands for a metal ion, in the second case, metal-exchange equilibrium is competition between ions metal for the ligand. The predominant process will be the formation of the most durable complex. For example, the solution contains ions: magnesium, zinc, iron (III), copper, chromium (II), iron (II) and manganese (II). When a small amount of ethylenediaminetetraacetic acid (EDTA) is introduced into this solution, competition between metal ions and binding to the iron (III) complex occurs, since it forms the most stable complex with EDTA.

The body constantly interacts with biometals (Mb) and bioligands (Lb), the formation and destruction of vital biocomplexes (MbLb):

In the human body, animals and plants, there are various mechanisms of protection and support of this balance from various xenobiotics (foreign substances), including heavy metal ions. Heavy metal ions not bound into a complex and their hydroxo complexes are toxic particles (MT). In these cases, along with the natural metal-ligand equilibrium, a new equilibrium can arise, with the formation of stronger foreign complexes containing toxicant metals (МтLb) or toxicant ligands (МbLt), which do not fulfill

necessary biological functions. When exogenous toxic particles enter the body, combined equilibria arise and, as a consequence, competition between processes. The predominant process will be the one that leads to the formation of the most durable complex compound:

Violations of metal-ligand homeostasis cause metabolic disorders, inhibit the activity of enzymes, destroy important metabolites such as ATP, cell membranes, and disrupt the gradient of ion concentration in cells. Therefore, artificial defense systems are being created. Chelation therapy (complex therapy) takes its due place in this method.

Chelation therapy is the elimination of toxic particles from the body, based on their chelation with s-element complexonates. Drugs used to remove toxic particles incorporated in the body are called detoxifiers.(Lg). Chelation of toxic particles with metal complexonates (Lg) converts toxic metal ions (MT) into non-toxic (MTLg) bound forms suitable for isolation and permeation through membranes, transport and excretion from the body. They retain the chelating effect in the body both in terms of the ligand (complexone) and the metal ion. This provides the body with metal-ligand homeostasis. Therefore, the use of complexonates in medicine, animal husbandry, plant growing provides detoxification of the body.

The basic thermodynamic principles of chelation therapy can be formulated in two positions.

I. The detoxifier (Lg) must effectively bind toxic ions (Mt, Lt), the newly formed compounds (MtLg) must be stronger than those that existed in the body:

II. The detoxifier should not destroy vital complex compounds (MBLb); compounds that can be formed during the interaction of a detoxifier and biometal ions (MBLg) should be less strong than those existing in the body:

7.11. APPLICATION OF COMPLEXONS AND COMPLEXONATES IN MEDICINE

Complexone molecules practically do not undergo cleavage or any change in the biological environment, which is their important pharmacological feature. Complexones are insoluble in lipids and readily soluble in water, therefore they do not penetrate or poorly penetrate through cell membranes, and therefore: 1) are not excreted by the intestines; 2) absorption of complexing agents occurs only when they are injected (only penicillamine is taken orally); 3) complexones circulate in the body mainly in the extracellular space; 4) excretion from the body is carried out mainly through the kidneys. This process is fast.

Substances that eliminate the effects of poisons on biological structures and inactivating poisons by chemical reactions are called antidotes.

One of the first antidotes used in chelation therapy is British anti-Lewisite (BAL). Currently used unitiol:

This drug effectively removes arsenic, mercury, chromium and bismuth from the body. Complexones and complexonates are most widely used for poisoning with zinc, cadmium, lead and mercury. Their use is based on the formation of stronger complexes with metal ions than complexes of the same ions with sulfur-containing groups of proteins, amino acids and carbohydrates. For lead elimination, EDTA-based preparations are used. The introduction into the body in large doses of drugs is dangerous, since they bind calcium ions, which leads to disruption of many functions. Therefore, apply tetacin(CaNa 2 EDTA), which is used to remove lead, cadmium, mercury, yttrium, cerium and other rare earth metals and cobalt.

Since the first therapeutic use of tetacin in 1952, this drug has found widespread use in the clinic of occupational diseases and continues to be an indispensable antidote. The mechanism of action of tetacin is quite interesting. Ions-toxicants displace the coordinated calcium ion from thetacin due to the formation of stronger bonds with oxygen and EDTA. The calcium ion, in turn, displaces the two remaining sodium ions:

Tetacin is injected into the body in the form of a 5-10% solution, the basis of which is a physiological solution. So, already after 1.5 hours after intraperitoneal injection, 15% of the administered dose of tetacin remains in the body, after 6 hours - 3%, and after 2 days - only 0.5%. The drug acts effectively and quickly when using the inhalation method of thetacin administration. It is rapidly absorbed and circulates in the blood for a long time. In addition, tetacin is used to protect against gas gangrene. It inhibits the action of zinc and cobalt ions, which are activators of the enzyme lecithinase, which is a gas gangrene toxin.

The binding of toxicants with tetacin in a low-toxic and more durable chelate complex, which is not destroyed and is easily excreted from the body through the kidneys, provides detoxification and balanced mineral nutrition. Close in structure and composition to pre-

paratam EDTA is the sodium-calcium salt of diethylenetriamine-pentaacetic acid (CaNa 3 DTPA) - pentacin and sodium salt of dacid (Na 6 DTPP) - trimefa-cyn. Pentacin is used mainly for poisoning with compounds of iron, cadmium and lead, as well as for the removal of radionuclides (technetium, plutonium, uranium).

Sodium salt of ethylenediamine diisopropylphosphonic acid (CaNa 2 EDTP) phosphicin successfully used to remove mercury, lead, beryllium, manganese, actinides and other metals from the body. Complexonates are very effective in removing some toxic anions. For example, cobalt (II) ethylenediamine tetraacetate, which forms a mixed ligand complex with CN -, can be recommended as an antidote for cyanide poisoning. A similar principle underlies the methods for removing toxic organic substances, including pesticides, containing functional groups with donor atoms capable of interacting with the complexonate metal.

An effective drug is succimer(dimercaptosuccinic acid, dimercaptosuccinic acid, hemet). It firmly binds almost all toxicants (Hg, As, Pb, Cd), but removes the ions of biogenic elements (Cu, Fe, Zn, Co) from the body, therefore it is almost never used.

Phosphorus complexonates are potent inhibitors of the crystal formation of calcium phosphates and oxalates. As an anti-calcifying drug in the treatment urolithiasis proposed ksidiphon - potassium-sodium salt of HEDP. Diphosphonates, in addition, in minimal doses, increase the incorporation of calcium into the bone tissue, prevent its pathological release from the bones. HEDP and other diphosphonates prevent various types of osteoporosis, including renal osteodystrophy, periodontal

destruction, as well as destruction of the transplanted bone in animals. The antiatherosclerotic effect of HEDP has also been described.

A number of diphosphonates, in particular HEDP, have been proposed in the United States as pharmaceuticals for the treatment of humans and animals suffering from metastatic bone cancer. By regulating the permeability of membranes, diphosphonates facilitate the transport of anticancer drugs into the cell, and therefore effective treatment various oncological diseases.

One of the pressing problems modern medicine is the task of express diagnostics of various diseases. In this aspect, a new class of preparations containing cations capable of performing the functions of a probe - radioactive magnetic relaxation and fluorescent labels - is of undoubted interest. Radioisotopes of certain metals are used as the main components of radiopharmaceuticals. Chelation of the cations of these isotopes with chelating agents makes it possible to increase their toxicological acceptability for the organism, to facilitate their transportation, and to ensure, within certain limits, the selectivity of concentration in certain organs.

The given examples by no means exhaust the whole variety of forms of application of complexonates in medicine. So, the dipotassium salt of magnesium ethylenediaminetetraacetate is used to regulate the fluid content in tissues in case of pathology. EDTA is used as a part of anticoagulant suspensions used in the separation of blood plasma, as a stabilizer of adenosine triphosphate in the determination of glucose in the blood, in the lightening and storage of contact lenses. In the treatment of rheumatoid diseases, diphosphonates are widely used. They are especially effective as anti-arthritic agents in combination with anti-inflammatory agents.

7.12. COMPLEXES WITH MACROCYCLIC COMPOUNDS

Among natural complex compounds, a special place is occupied by macrocomplexes based on cyclic polypeptides containing internal cavities of a certain size, in which there are several oxygen-containing groups capable of binding the cations of those metals, including sodium and potassium, the sizes of which correspond to the dimensions of the cavity. Such substances, being in a biological

Rice. 7.2. Valinomycin K + Ion Complex

chemical materials, provide the transport of ions across membranes and are therefore called ionophores. For example, valinomycin transports a potassium ion across the membrane (Figure 7.2).

With the help of another polypeptide - gramicidin A the transport of sodium cations is carried out by the relay mechanism. This polypeptide is folded into a "tube", the inner surface of which is lined with oxygen-containing groups. The result is

a sufficiently long hydrophilic channel with a certain section corresponding to the size of the sodium ion. Sodium ion, entering the hydrophilic channel from one side, is transferred from one to the other oxygen groups, like a relay race along an ion-conducting channel.

So, a cyclic polypeptide molecule has an intramolecular cavity, into which a substrate of a certain size and geometry can enter according to the principle of a key and a lock. The cavity of such internal receptors is bordered by active centers (endoreceptors). Depending on the nature of the metal ion, non-covalent interaction (electrostatic, hydrogen bonding, van der Waals forces) with alkali metals and covalent with alkaline earth metals can occur. As a result of this, supramolecules- complex associates consisting of two or more particles held together by intermolecular forces.

The most common in living nature are tetradentate macrocycles - porphins and corrinoids, similar in structure. The tetradent cycle can be schematically represented in the following form (Fig. 7.3), where the arcs mean the same type of carbon chains connecting donor nitrogen atoms in a closed cycle; R 1, R 2, R 3, P 4 are hydrocarbon radicals; M n + is a metal ion: in chlorophyll there is an ion of Mg 2+, in hemoglobin an ion of Fe 2+, in hemocyanin an ion of Cu 2+, in vitamin B 12 (cobalamin) an ion of Co 3+.

Donor nitrogen atoms are located at the corners of a square (indicated by a dotted line). They are tightly coordinated in space. That's why

porphyrins and corrinoids form strong complexes with cations of various elements and even alkaline earth metals. It is essential that regardless of the denticity of the ligand, the chemical bond and the structure of the complex are determined by donor atoms. For example, complexes of copper with NH 3, ethylenediamine, and porphyrin have the same square structure and similar electronic configuration. But polydentate ligands bind to metal ions much more strongly than monodentate ligands

Rice. 7.3. Tetradentate macrocycle

with the same donor atoms. The strength of ethylenediamine complexes is 8-10 orders of magnitude higher than the strength of the same metals with ammonia.

Bioinorganic complexes of metal ions with proteins are called bioclusters - complexes of metal ions with macrocyclic compounds (Fig. 7.4).

Rice. 7.4. Schematic representation of the structure of bioclusters of certain sizes of protein complexes with d-element ions. Types of interactions of a protein molecule. M n + - metal ion of the active center

There is a cavity inside the biocluster. It includes a metal that interacts with donor atoms of binding groups: OH -, SH -, COO -, -NH 2, proteins, amino acids. The most famous metallofer-

cents (carbonic anhydrase, xanthine oxidase, cytochromes) are bioclusters, the cavities of which form the centers of enzymes containing Zn, Mo, Fe, respectively.

7.13. MULTINUCLEAR COMPLEXES

Heterovalent and heteronuclear complexes

Complexes containing several central atoms of one or different elements are called multi-core. The possibility of the formation of multinuclear complexes is determined by the ability of some ligands to bind with two or three metal ions. Such ligands are called bridge. Respectively bridge complexes are also called. Monatomic bridges are also possible in principle, for example:

They use lone electron pairs belonging to the same atom. The role of bridges can be played by polyatomic ligands. Such bridges use lone electron pairs belonging to different atoms polyatomic ligand.

A.A. Greenberg and F.M. Filinov investigated bridging compounds of the composition in which the ligand binds complex compounds of the same metal, but in different oxidation states. G. Taube named them electron transfer complexes. He investigated the reactions of electron transfer between the central atoms of various metals. Systematic studies of the kinetics and mechanism of redox reactions have led to the conclusion that electron transfer between two complexes of pro

comes through the formed ligand bridge. The exchange of an electron between 2 + and 2 + occurs through the formation of an intermediate bridging complex (Fig. 7.5). Electron transfer occurs through the chloride bridging ligand, resulting in the formation of 2 + complexes; 2+.

Rice. 7.5. Electron transfer in an intermediate multinuclear complex

A wide variety of polynuclear complexes have been obtained through the use of organic ligands containing several donor groups. The condition for their formation is such an arrangement of donor groups in the ligand that does not allow the chelate rings to close. There are frequent cases when a ligand has the ability to close the chelate cycle and simultaneously act as a bridging one.

The active principle of electron transfer is transition metals, which exhibit several stable oxidation states. This gives titanium, iron and copper ions ideal electron carrier properties. The set of options for the formation of heterovalent (HVC) and heteronuclear complexes (HNC) based on Ti and Fe is shown in Fig. 7.6.

The reaction

Reaction (1) is called cross reaction. In exchange reactions, the intermediate will be heterovalent complexes. All theoretically possible complexes are actually formed in solution under certain conditions, which has been proven by various physicochemical

Rice. 7.6. Formation of heterovalent complexes and heteronuclear complexes containing Ti and Fe

methods. For the transfer of electrons, the reactants must be in states close in energy. This requirement is called the Franck-Condon principle. The transfer of an electron can occur between the atoms of the same transition element, which are in different oxidation states of the HVC, or between different HNC elements, the nature of the metallocenters of which is different. These compounds can be defined as electron transfer complexes. They are convenient carriers of electrons and protons in biological systems. The addition and release of an electron causes changes in only the electronic configuration of the metal, without changing the structure of the organic component of the complex. All these elements have several stable oxidation states (Ti +3 and +4; Fe +2 and +3; Cu +1 and +2). In our opinion, nature has given these systems a unique role to ensure the reversibility of biochemical processes with minimal energy consumption. Reversible reactions include reactions with thermodynamic and thermochemical constants from 10 -3 to 10 3 and with a small value of ΔG o and E o processes. Under these conditions, the starting materials and reaction products can be in comparable concentrations. By changing them in a certain range, it is easy to achieve the reversibility of the process; therefore, in biological systems, many processes are of an oscillatory (wave) nature. Redox systems containing the above pairs cover a wide range of potentials, which allows them to enter into interactions accompanied by moderate changes in Δ Go and E °, with many substrates.

The likelihood of HVA and HNA formation increases significantly when the solution contains potentially bridging ligands, i.e. molecules or ions (amino acids, hydroxy acids, chelating agents, etc.) capable of binding two metal centers at once. The possibility of electron delocalization in the HVC helps to reduce the total energy of the complex.

A more realistic set of possible variants of the formation of GWC and HNC, in which the nature of the metal centers is different, is seen in Fig. 7.6. A detailed description of the formation of HVC and HNC and their role in biochemical systems are considered in the works of A.N. Glebova (1997). Redox pairs must be structurally adjusted to each other, then the transfer becomes possible. By selecting the components of the solution, it is possible to "lengthen" the distance over which the electron is transferred from the reducing agent to the oxidizing agent. With the coordinated movement of particles, an electron can be transferred over long distances by a wave mechanism. As a "corridor" can be a hydrated protein chain, etc. The probability of electron transfer at a distance of up to 100A is high. The length of the "corridor" can be increased by additives (alkali metal ions, background electrolytes). This opens up great opportunities in the field of control over the composition and properties of GWC and HNC. In solutions, they play the role of a kind of "black box" filled with electrons and protons. Depending on the circumstances, he can give them to other components or replenish his "stocks". The reversibility of reactions with their participation makes it possible to repeatedly participate in cyclic processes. Electrons move from one metal center to another, oscillate between them. The molecule of the complex remains asymmetrical and can take part in redox processes. GVK and GNK actively participate in oscillatory processes in biological media. This type of reaction is called oscillatory reactions. They are found in enzymatic catalysis, protein synthesis, and other biochemical processes accompanying biological phenomena. This includes periodic processes of cellular metabolism, waves of activity in cardiac tissue, in brain tissue, and processes occurring at the level of ecological systems. An important stage of metabolism is the elimination of hydrogen from nutrients. At the same time, hydrogen atoms pass into the ionic state, and the electrons separated from them enter the respiratory chain and give up their energy to the formation of ATP. As we have established, titanium complexonates are active carriers of not only electrons, but also protons. The ability of titanium ions to play their role in the active center of enzymes such as catalases, peroxidases, and cytochromes is determined by its high ability to form complexes, form the geometry of a coordinated ion, form multinuclear HVAs and HNAs of various compositions and properties in function of pH, concentration of the transition element Ti and the organic component of the complex. their molar ratio. This ability is manifested in an increase in the selectivity of the complex

in relation to substrates, products of metabolic processes, activation of bonds in the complex (enzyme) and the substrate by coordinating and changing the shape of the substrate in accordance with the steric requirements of the active center.

Electrochemical transformations in the body associated with the transfer of electrons are accompanied by a change in the oxidation state of particles and the appearance of a redox potential in solution. An important role in these transformations belongs to the multinuclear complexes GVK and GNK. They are active regulators of free radical processes, a system for the utilization of reactive oxygen species, hydrogen peroxide, oxidants, radicals and are involved in the oxidation of substrates, as well as in maintaining antioxidant homeostasis, in protecting the body from oxidative stress. Their enzymatic effect on biosystems is similar to enzymes (cytochrome, superoxide dismutase, catalase, peroxidase, glutathione reductase, dehydrogenases). All this indicates the high antioxidant properties of complexonates of transition elements.

7.14. QUESTIONS AND TASKS FOR SELF-CHECK PREPARATION FOR EXERCISES AND EXAMINATIONS

1.Give the concept of complex compounds. How are they different from double salts, and what do they have in common?

2. Make up the formulas of complex compounds by their name: ammonium dihydroxotetrachloroplatinate (IV), triamminthrinitroco-balt (III), give their characteristics; indicate the internal and external coordination area; central ion and its oxidation state: ligands, their number and dentition; the nature of the connections. Write the equation for dissociation in aqueous solution and an expression for the stability constant.

3.General properties of complex compounds, dissociation, stability of complexes, Chemical properties complexes.

4. How is the reactivity of the complexes characterized from thermodynamic and kinetic positions?

5. Which amino complexes will be more durable than tetraamino-copper (II), and which ones will be less strong?

6. Give examples of macrocyclic complexes formed by alkali metal ions; ions of d-elements.

7. On what basis are complexes classified as chelated? Give examples of chelated and non-chelated complex compounds.

8. Using the example of copper glycinate, give the concept of intracomplex compounds. Write the structural formula of magnesium complexonate with ethylenediaminetetraacetic acid in sodium form.

9. Give a schematic structural fragment of a polynuclear complex.

10. Give the definition of polynuclear, heteronuclear and heterovalent complexes. The role of transition metals in their formation. The biological role of these components.

11. What types of chemical bonds are found in complex compounds?

12. List the main types of hybridization of atomic orbitals that can occur for the central atom in the complex. What is the geometry of the complex depending on the type of hybridization?

13. Based on the electronic structure of the atoms of the elements of s-, p- and d-blocks, compare the ability to form complexes and their place in the chemistry of complexes.

14. Give the definition of chelating agents and chelating agents. Give examples of the most used in biology and medicine. Give the thermodynamic principles on which chelation therapy is based. The use of complexonates for neutralization and elimination of xenobiotics from the body.

15. Consider the main cases of violation of metal-ligand homeostasis in the human body.

16. Give examples of biocomplex compounds containing iron, cobalt, zinc.

17. Examples of competing processes involving hemoglobin.

18. The role of metal ions in enzymes.

19. Explain why for cobalt in complexes with complex (polydentate) ligands, oxidation state +3 is more stable, and in ordinary salts, such as halides, sulfates, nitrates, oxidation state +2?

20. For copper, the oxidation states +1 and +2 are characteristic. Can copper catalyze electron transfer reactions?

21. Can zinc catalyze redox reactions?

22. What is the mechanism of action of mercury as a poison?

23. Indicate the acid and base in the reaction:

AgNO 3 + 2NH 3 = NO 3.

24. Explain why sodium hydroxyethylidene diphosphonic acid potassium salt is used as a drug, and not HEDP.

25. How is the transport of electrons in the body carried out with the help of metal ions, which are part of biocomplex compounds?

7.15. TEST PROBLEMS

1. The oxidation state of the central atom in the complex ion is 2- is equal to:

a) -4;

b) +2;

in 2;

d) +4.

2. Most stable complex ion:

a) 2-, Kn = 8.5x10 -15;

b) 2-, Kn = 1.5x10 -30;

c) 2-, Kn = 4x10 -42;

d) 2-, Kn = 1x10 -21.

3. The solution contains 0.1 mol of the PtCl 4 4NH 3 compound. Reacting with AgNO 3, it forms 0.2 mol of AgCl precipitate. Give the starting substance the coordination formula:

a) Cl;

b) Cl 3;

c) Cl 2;

d) Cl 4.

4. What form do the complexes formed as a result of sp 3 d 2-gi- breeding?

1) tetrahedron;

2) squares;

4) trigonal bipyramid;

5) linear.

5. Find the formula for the pentaammine chlorocobalt (III) sulfate compound:

a) Na 3 ;

6) [CoCl 2 (NH 3) 4] Cl;

c) K 2 [Co (SCN) 4];

d) SO 4;

e) [Co (H 2 O) 6] C1 3.

6. Which ligands are polydentate?

a) C1 -;

b) H 2 O;

c) ethylenediamine;

d) NH 3;

e) SCN -.

7. Complexing agents are:

a) atoms donors of electron pairs;

c) atoms and ions-acceptors of electron pairs;

d) atoms and ions donors of electron pairs.

8. The elements have the least complexing ability:

a) s; c) d;

b) p; d) f

9. Ligands are:

a) electron pair donor molecules;

b) ions-acceptors of electron pairs;

c) molecules and ions-donors of electron pairs;

d) molecules and ions-acceptors of electron pairs.

10. Communication in the internal coordination area of ​​the complex:

a) covalent exchange;

b) covalent donor-acceptor;

c) ionic;

d) hydrogen.

11. The best complexing agent will be:

The reactions of coordination compounds always occur in the coordination sphere of the metal with the ligands bound in it. Therefore, it is obvious that in order for anything to happen at all, ligands must be able to get into this sphere. This can happen in two ways:

• a coordination unsaturated complex binds a new ligand
• in the already completed coordination sphere, one ligand is changed to another.

We already got acquainted with the first method when we discussed coordination unsaturation and the 18-electron rule. We will deal with the second here.

Any combination of ligands of any type can be substituted

But usually there is an unspoken rule - the number of occupied coordination places does not change. In other words, the substitution does not change the electron count. Substitution of a ligand of one type for another is quite possible and often occurs in reality. Let us only pay attention to the correct handling of charges when the L-ligand is changed to the X-ligand and vice versa. If we forget about this, then the oxidation state of the metal will change, and the substitution of ligands is not a redox process (if you find or come up with a nasty example, let me know - automatic set-off right away, if I cannot prove that you were mistaken, why even in In this case, I guarantee a positive contribution to karma).

Substitution involving hapto-ligands

With more complex ligands, there is no more difficulty - you just need to remember the rather obvious rule: the number of ligand sites (that is, the total number of ligands or ligand centers of the X- or L-types) is preserved. This follows directly from the preservation of the electron count. Here are some self-explanatory examples.

Let's take a look at the last example. The starting reagent for this reaction is iron dichloride FeCl 2. Until recently, we would have said: "It's just salt, where does the coordination chemistry?" But we will no longer allow ourselves such ignorance. In the chemistry of transition metals, there are no “just salts”; any derivatives are coordination compounds to which all reasoning about electrons, d-configuration, coordination saturation, etc. is applicable. Iron dichloride, as we used to write it, would turn out to be a Fe (2+) complex of the MX 2 type with the d 6 configuration and the number of electrons 10. Not enough! Fine? After all, we have already figured out that ligands are implicit. To make a reaction, we need a solvent, and for such reactions it is most likely THF. The dissolution of the crystalline iron salt in THF occurs precisely because the donor solvent takes up free spaces, and the energy of this process compensates for the destruction of the crystal lattice. We would not be able to dissolve this "salt" in a solvent that does not provide metal solvation services due to the Lewis basicity. In this case, and in a million others, solvation is simply a coordination interaction. Let us write, just for definiteness, the result of solvation in the form of the FeX 2 L 4 complex, in which two chlorine ions remain in the coordination sphere in the form of two X ligands, although most likely they are also displaced by donor solvent molecules with the formation of a charged complex FeL 6 2+... In this case, it is not so important. And so and so, we can safely assume that we have an 18-electronic complex both on the left and on the right.

Substitution, addition and dissociation of ligands are closely and inextricably linked

If we remember organic chemistry, then there were two mechanisms of substitution at a saturated carbon atom - SN1 and SN2. In the first, the substitution took place in two stages: the old substitute first left, leaving a vacant orbital on the carbon atom, which was followed by a new substituent with a pair of electrons. The second mechanism assumed that departure and arrival were carried out simultaneously, in a coordinated manner, and the process was one-step.

In the chemistry of coordination compounds, it is quite possible to imagine something similar. But a third possibility appears, which the saturated carbon atom did not have - first we attach a new ligand, then we unhook the old one. It immediately becomes clear that this third option is hardly possible if the complex already has 18 electrons and is coordinatively saturated. But it is quite possible if the number of electrons is 16 or less, that is, the complex is unsaturated. Let us immediately recall the obvious analogy from organic chemistry - nucleophilic substitution at an unsaturated carbon atom (in an aromatic ring or at a carbonyl carbon) also proceeds first as the addition of a new nucleophile, and then the elimination of the old one.

So, if we have 18 electrons, then the substitution proceeds as a cleavage-attachment (fans of “smart” words use the term dissociative-associative or simply dissociative mechanism). Another way would require the expansion of the coordination sphere to a count of 20 electrons. This is not absolutely impossible, and such options are sometimes even considered, but it is definitely very unprofitable and every time in case of suspicion of such a path, very strong evidence is required. In most of these stories, researchers eventually come to the conclusion that they have overlooked or left out something, and the associative mechanism has been rejected. So, if the original complex with 18 electrons, then first one ligand must leave, then a new one should come in its place, for example:

If we want to introduce into the coordination sphere a hapto-ligand occupying several places, then first we must free them all. As a rule, this occurs only under sufficiently severe conditions, for example, in order to replace three carbonyls with η 6 -benzene in the chromium carbonyl, the mixture is heated for many hours under pressure, from time to time releasing the released carbon monoxide. Although the scheme depicts the dissociation of three ligands with the formation of a very unsaturated complex with 12 electrons, in reality, the reaction most likely occurs in stages, one carbonyl is removed, and benzene enters the sphere, gradually increasing the haptiness, through the stages minus CO - dihapto - minus one more CO - tetragapto - minus one more CO - hexagapto, so that less than 16 electrons are not obtained.

So, if we have a complex with 16 electrons or less, then the ligand substitution, most likely, proceeds as an addition-elimination (for those who like thoughtful words: associative-dissociative or simply associative): the new ligand comes first, then the old one leaves. Two obvious questions arise: why does the old ligand leave, because 18 electrons is very good, and why not do the opposite in this case, as in 18-electron complexes. The first question is easy to answer: each metal has its own habits, and some metals, especially from the late ones, with almost completely filled d-shells, prefer 16-electron counting and the corresponding structural types, and therefore throw out an extra ligand, returning to their favorite configuration. Sometimes the space factor still interferes with the matter, the already existing ligands are large and the additional one feels like a bus passenger at rush hour. It's easier to get off and walk on foot than to suffer like that. However, you can shove out another passenger, let him take a walk, and we'll go. The second question is also simple - in this case, the dissociative mechanism would first have to give a 14-electron complex, and this is rarely beneficial.

Here's an example. For a change, we will replace the X-ligand with the L-ligand, and we will not get confused in the oxidation states and charges. Once again: upon substitution, the oxidation state does not change, and if the X-ligand is gone, then the loss must be compensated for by the charge on the metal. If we forget about this, then the oxidation state would decrease by 1, and this is not true.

And one more oddity. A metal-pyridine bond was formed due to the lone pair on nitrogen. In organic chemistry, in this case, we would definitely show a plus on pyridine nitrogen (for example, during protonation or the formation of a quaternary salt), but we never do this in coordination chemistry either with pyridine or with any other L-ligands. This is terribly annoying for everyone who is used to the strict and unambiguous system of drawing structures in organic chemistry, but it will take some getting used to, it is not that difficult.

And there is no exact analogue of SN2 in the chemistry of coordination compounds, there is a distant one, but it is relatively rare and we do not really need it.

Stable and labile ligands

It would be possible not to talk about the mechanisms of ligand substitution at all, if it were not for one extremely important circumstance that we will use a lot: ligand substitution, be it associative or dissociative, necessarily presupposes the dissociation of the old ligand. And it is very important for us to know which ligands leave easily and which ones leave poorly, preferring to remain in the coordination sphere of the metal.

As we will soon see, in any reaction, some of the ligands remain in the coordination sphere and do not change. Such ligands are usually called spectator ligands (if you do not want such simple, “unscientific” words, use the English word spectator in the local transcription spectator, spectator ligand, but, I beg you, not spectator - it's unbearable!). And a part is directly involved in the reaction, turning into reaction products. Such ligands are called actors (not actors!), That is, acting. It is quite clear that the ligand-actors need to be easily introduced and removed into the coordination sphere of the metal, otherwise the reaction will simply get stuck. But it is better to leave ligands-spectators in the coordination sphere for many reasons, but at least for such a banal one as the need to avoid unnecessary fuss around the metal. It is better that only ligands and actors and in the required quantities can participate in the required process. If there are more available coordination sites than necessary, excess ligand-actors can settle on them, and even those that will participate in side reactions, reducing the yield of the target product and selectivity. In addition, spectator ligands almost always perform many important functions, for example, provide the solubility of complexes, stabilize the correct valence state of the metal, especially if it is not quite usual, help individual steps, provide stereoselectivity, etc. We are not decoding yet, because we will discuss all this in detail when we get to specific reactions.

It turns out that some of the ligands in the coordination sphere should be firmly bound and not prone to dissociation and substitution by other ligands. Such ligands are usually called coordinatively stable ... Or simply stable, if it is clear from the context that we are talking about the bond strength of the ligands, and not about their own thermodynamic stability, which just does not bother us at all.

And ligands that easily and willingly enter and exit, and are always ready to give way to others, are called coordination labile , or simply labile, and here, fortunately, there are no ambiguities.

Cyclobutadiene as a ligand

This is probably the most striking example of the fact that in the coordination sphere a very unstable molecule can become an excellent ligand, and by definition, coordination stable, if only because if it dares to go out of the warm and cozy sphere outside, nothing good awaits it (at the cost the way out is just the energy of anti-aromatic destabilization).

Cyclobutadiene and its derivatives are the most famous examples antiaromatic. These molecules exist only at low temperatures, and in a highly distorted form - in order to go as far as possible from antiaromaticity, the cycle is distorted into an elongated rectangle, removing delocalization and maximally weakening the conjugation of double bonds (otherwise this is called the Jahn-Teller effect of the second kind: degenerate system, and cyclobutadiene-square is a degenerate biradical, remember the Frost circle - it is distorted and reduces symmetry to remove the degeneracy).

But in the complexes, cyclobutadiene and substituted cyclobutadienes are excellent tetragapto ligands, and the geometry of such ligands is exactly a square, with the same bond lengths. How and why this happens is a separate story, and not nearly as obvious as it is often presented.

Coordination labile ligands

It should be understood that there is no reinforced concrete fence with barbed wire and guard towers between the areas of labile and stable ligands. Firstly, it depends on the metal, and in this context, ZhMKO works well. For example, late transition metals prefer soft ligands, while early transition metals prefer hard ligands. For example, iodide is very tightly attached to the d 8 atoms of palladium or platinum, but rarely even enter the coordination sphere of titanium or zirconium in the d 0 configuration. But in many metal complexes with less pronounced features, iodide manifests itself as a completely labile ligand, easily giving way to others.

All other things being equal:

• L-ligands are generally more labile than X-ligands;
• the lability of the X-ligands is determined by the hardness / softness and nature of the metal;
• “Implicit” ligands are very labile: solvents and bridges in dimers and clusters, so much so that their presence in the coordination sphere is often generally neglected and structures without them with a formally unsaturated coordination sphere are drawn;
• dihapto ligands such as alkenes and alkynes behave like typical L ligands: they are usually quite labile;
• ligands with greater haptiness are rarely labile, but if the polyhapto ligand can change the mode of binding to mono-hapto, it becomes more labile, as, for example, η 3 -allyls behave like this;
• chelating ligands forming 5 and 6-membered chelate rings are stable, while chelates with fewer or more ring atoms are labile, at least in one center (the chelate ring opens and the ligand remains hanging as a simple one). This is how, for example, acetate behaves;

Coordination stable ligands

Let's repeat it all one more time, only from the other side

In the coordination sphere, metals are preserved (are coordinatively stable) as a rule:

• 5- and 6-membered chelators;
• polyhapto ligands: in order to knock out cyclopentadienyls or benzene (arenas) from the coordination sphere, all sorts of special techniques have to be used - they just do not come out, often withstanding even prolonged heating;
• metal-bound ligands with a high proportion of π-donor effect (back-donation);
• soft ligands in late transition metals;
• The “last” ligand in the coordination sphere.

The latter condition looks strange, but imagine a complex that has many different ligands, among which there are no unconditionally stable ones (there are no chelators and polyhapto ligands). Then the ligands in the reactions will change, relatively speaking, in the order of relative lability. The least labile and will remain the last. This trick takes place, for example, when we use palladium phosphine complexes. Phosphines are relatively stable ligands, but when there are many of them, and the metal is rich in electrons (d 8, d 10), they give way, one by one, to ligand-actors. But the last phosphine ligand usually remains in the coordination sphere, and this is very good from the point of view of the reactions in which these complexes are involved. We will return to this important issue later. Here is a fairly typical example, when from the initial coordination sphere of the phosphine palladium complex in the Heck reaction, only one remains, the “last” phosphine. This example brings us very close to the most important concept in transition metal complex reactions - the ligand control concept. We will discuss it later.

Remetalling

When replacing some ligands with others, it is important not to overdo it with the reactivity of the incoming ligand. When we deal with the reactions of organic molecules, it is important for us to deliver exactly one molecule of each of the reagents to the coordination sphere. If instead of one there are two molecules, there is a high probability of side reactions involving two identical ligands. A loss of reactivity is also possible due to saturation of the coordination sphere and the impossibility of introducing into it other ligands necessary for the expected process. This problem occurs especially often when strong anionic nucleophiles, for example, carbanions, are introduced into the coordination sphere. To avoid this, less reactive derivatives are used, in which, instead of an alkali metal cation, which causes a high ionicity of the bond, less electropositive metals and metalloids (zinc, tin, boron, silicon, etc.) are used that form covalent bonds with the nucleophilic part ... The reactions of such derivatives with transition metal derivatives give ligand substitution products, in principle, just as if the nucleophile were in the anionic form, but due to reduced nucleophilicity with fewer complications and without side reactions.

Such ligand substitution reactions are commonly called transmetallation, in order to emphasize the obvious circumstance that the nucleophile seems to change metals from more electropositive to less electropositive. Thus, this name contains an element of unpleasant schizophrenia - we seem to have already agreed that we will look at all reactions from the point of view of a transition metal, but suddenly we broke off again and are looking at this reaction and only this reaction from the point of view of a nucleophile. We'll have to endure, this is how the terminology developed and is so accepted. In fact, this word goes back to the early chemistry of organometallic compounds and to the fact that the action of lithium or organomagnesium compounds on halides of various metals and metalloids is one of the main methods for the synthesis of any organometallic, primarily intransient, and the reaction that we are now considering in chemistry of coordination compounds of transition metals is just a generalization of the old method of organometallic chemistry, from which it all grew.

How does remetalling work?

Remetalling is both similar to conventional substitution and not. It seems that if we consider a non-transition organometallic reagent to be simply a carbanion with a counterion, that is, the carbon-non-transition metal bond is ionic. But this idea seems to be true only for the most electropositive metals - for magnesium. But even for zinc and tin, this idea is very far from the truth.

Therefore, two σ-bonds and four atoms at their ends enter into the reaction. As a result, two new σ-bonds are formed and four atoms are bonded to each other in a different order. Most likely, all this occurs simultaneously in a four-membered transition state, and the reaction itself has a consistent character, like very many other reactions of transition metals. The abundance of electrons and orbitals for literally all tastes and all kinds of symmetries makes transition metals capable of simultaneously maintaining bonds in transition states with several atoms.

In the case of remetalling, we get a special case of a very general process, which is simply called σ-bond metathesis. Do not confuse only with the real metathesis of olefins and acetylenes, which are complete catalytic reactions with their own mechanisms. In this case, we are talking about the mechanism of re-metallization or another process in which something similar occurs.