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 something to happen at all, ligands must be able to fall into this sphere. This can happen in two ways:

• the coordinatively unsaturated complex binds the new ligand
• in an already completed coordination sphere, one ligand changes to another.

We already got acquainted with the first method when we discussed coordination unsaturation and the 18-electron rule. Let's do the second one here.

Ligands of any type can be substituted in any combination

But usually there is an unspoken rule - the number of occupied coordination places does not change. In other words, substitution does not change the electron count. The 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 changes to the X-ligand and vice versa. If we forget about this, then the degree of oxidation of the metal will change, and the replacement of ligands is not a redox process (if you find or come up with a nasty example, let me know - the offset will be automatic immediately 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 a fairly obvious rule: the number of ligand sites (that is, the total number of ligands or ligand centers of X- or L-types) is conserved. This follows directly from the conservation of the electron count. Here are some self-evident examples.

Let's 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, what does the coordination chemistry have to do with it?”. But we will no longer allow ourselves such ignorance. In the chemistry of transition metals, there are no “simply salts”, any derivatives are coordination compounds, to which all the arguments about electron counting, d-configuration, coordination saturation, etc. are applicable. Iron dichloride, as we are used to writing it, would be an MX 2 type Fe(2+) complex with a d 6 configuration and 10 electrons. 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 occupies free places, 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 Lewis basicity. In this case, and in a million others like it, solvation is just a coordination interaction. Let us write, just for definiteness, the result of solvation in the form of a 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's not that important. And so, and so we can safely assume that we have an 18-electron complex 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 substitution mechanisms at a saturated carbon atom - SN1 and SN2. In the first, the substitution occurred in two stages: the old substituent 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 the departure and arrival are carried out simultaneously, in concert, and the process was one-stage.

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 an obvious analogy from organic chemistry - nucleophilic substitution at an unsaturated carbon atom (in an aromatic ring or carbonyl carbon) also goes 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 goes like a split-off-attachment (fans of “smart” words use the term dissociative-associative or simply dissociative mechanism). Another way would require expanding 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 disadvantageous and every time if such a path is suspected, very weighty evidence is required. In most of these stories, the researchers eventually came to the conclusion that they overlooked or did not take into account something, and the associative mechanism was rejected. So, if the original complex has 18 electrons, then first one ligand must leave, then a new one should come in its place, for example:

If we want to introduce a hapto ligand that occupies several positions into the coordination sphere, we must first release 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 chromium carbonyl, the mixture is heated under pressure for many hours, from time to time bleed off 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, leaving one carbonyl, and benzene enters the sphere, gradually increasing the hapticity, through the stages minus CO - dihapto - minus one more CO - tetrahapto - 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-detachment (for lovers of thoughtful words: associative-dissociative or simply associative): the new ligand first comes, 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 late ones with almost completely filled d-shells, prefer the 16-electron count and the corresponding structural types, and therefore discard the extra ligand, returning to their favorite configuration. Sometimes the spatial factor also 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 around on foot than to suffer like that. However, you can push another passenger out, 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 is an example. For a change, we will replace the X-ligand with an L-ligand, and we will not get confused in the oxidation states and charges. Once again: when substituting, the oxidation state does not change, and if the X-ligand is gone, then the loss must be compensated by the charge on the metal. If we forget about this, then the oxidation state would decrease by 1, which 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 necessarily show a plus on the nitrogen of pyridine (for example, during protonation or the formation of a quaternary salt), but we never do this in coordination chemistry with either pyridine or 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 you have to get used to it, it's 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 not for one extremely important circumstance, which we will use a lot: the substitution of ligands, whether associative or dissociative, necessarily implies the dissociation of the old ligand. And it is very important for us to know which ligands leave easily and which 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 a spectator - this is unbearable!). And part directly participates 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 must be easily introduced and removed into the coordination sphere of the metal, otherwise the reaction will simply get stuck. But spectator ligands are better left 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, actors and in the required quantities, can participate in the desired process. If there are more available coordination sites than necessary, extra ligands-actors can sit 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, ensure 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 do not decipher it 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 must be strongly bound and not prone to dissociation and substitution by other ligands. Such ligands are 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 leave, and are always ready to give way to others, are called coordinatively labile , or simply labile, and here, fortunately, there are no ambiguities.

Cyclobutadiene as a ligand

Here 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, coordinationally stable, if only because if it ventures out of the warm and cozy sphere, nothing good awaits it (at the cost of output is just the energy of anti-aromatic destabilization).

Cyclobutadiene and its derivatives are the best known examples of antiaromaticity. These molecules exist only at low temperatures, and in a highly distorted form - in order to get as far as possible from antiaromaticity, the cycle is distorted into an elongated rectangle, removing delocalization and weakening the conjugation of double bonds as much as possible (otherwise, this is called the Jahn-Teller effect of the 2nd kind: degenerate system, and cyclobutadiene square is a degenerate diradical, remember Frost's circle - distorted and reduces symmetry to remove degeneracy).

But in complexes, cyclobutadiene and substituted cyclobutadienes are excellent tetrahapto ligands, and the geometry of such ligands is precisely a square, with identical bond lengths. How and why this happens is a separate story, and far from being as obvious as it is often presented.

Coordination labile ligands

You need to understand that there is no reinforced concrete fence with barbed wire and guard towers between the areas of labile and stable ligands. First, it depends on the metal, and in this context, GMKO works well. For example, late transition metals prefer soft ligands, while early transition metals prefer hard ligands. For example, iodide clings very tightly to the d 8 atoms of palladium or platinum, but rarely even enters the coordination sphere of titanium or zirconium in the d 0 configuration. But in many metal complexes with not so pronounced features, iodide manifests itself as a completely labile ligand, easily giving way to others.

Other things being equal:

• L-ligands are generally more labile than X-ligands;
• the lability of 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 neglected altogether and structures are drawn without them with a formally unsaturated coordination sphere;
• digapto ligands, such as alkenes and alkynes, behave like typical L ligands: they are usually quite labile;
• ligands with a higher haptiness are rarely labile, but if a poly-hapto ligand can change the mode of bonding to a mono-hapto, it becomes more labile, for example, η 3 -allyls behave this way;
• chelate ligands forming 5- and 6-membered chelate rings are stable, while chelates with fewer or more ring atoms are labile at least at one center (the chelate ring opens and the ligand remains hanging as a simple one). This is how, for example, acetate behaves;

Coordinationally stable ligands

Let's do it again, but on the other side

In the coordination sphere of metals, as a rule, the following are preserved (are coordinately stable):

• 5 and 6-membered chelators;
• polyhapto-ligands: in order to knock cyclopentadienyls or benzene (arenes) out of the coordination sphere, all sorts of special tricks have to be used - they just don’t come out just like that, often withstanding even prolonged heating;
• ligands associated with the metal with a high proportion of the π-donor effect (back-donation);
• soft ligands in late transition metals;
• the “last” ligand in the coordination sphere.

The last condition looks strange, but imagine a complex that has many different ligands, among which there are no unconditionally stable ones (no chelators and polygapto-ligands). Then, in the reactions, the ligands will change, relatively speaking, in the order of relative lability. The least labile and will remain the last. Such a focus occurs, for example, when we use phosphine complexes of palladium. 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 participate. We will return to this important issue later. Here is a rather typical example, when only one “last” phosphine remains from the initial coordination sphere of the palladium phosphine complex in the Heck reaction. This example brings us very close to the most important concept in transition metal complex reactions, the concept of ligand control. We'll discuss later.

Remetalization

When substituting one ligand for another, it is important not to overdo it with the reactivity of the incoming ligand. When we are dealing with reactions of organic molecules, it is important for us to deliver exactly one molecule of each of the reagents to the coordination sphere. If two molecules enter instead of one, there is a high probability of side reactions involving two identical ligands. A loss of reactivity is also possible due to the saturation of the coordination sphere and the impossibility of introducing into it other ligands necessary for the expected process. This problem arises 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 bond ionicity, less electropositive metals and metalloids (zinc, tin, boron, silicon, etc.) are used that form covalent bonds with the nucleophilic part . Reactions of such derivatives with transition metal derivatives give ligand substitution products, in principle, exactly as if the nucleophile were in the anionic form, but due to reduced nucleophilicity with fewer complications and no side reactions.

Such ligand substitution reactions are usually called transmetallation to emphasize the obvious fact that the nucleophile seems to change metals - 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 the transition metal, but suddenly we broke down again and look at this reaction and only this reaction from the point of view of a nucleophile. We'll have to be patient, that's how the terminology has developed and that's how it is 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 the halides of various metals and metalloids is one of the main methods for the synthesis of any organometallic, primarily intransitive, and the reaction that we are now considering in chemistry of coordination compounds of transition metals is simply a generalization of the old method of organometallic chemistry, from which it all grew.

How does remetallization take place?

Remetaling is both similar to regular substitution and not. It seems that if we consider an intransitive organometallic reagent as just a carbanion with a counterion, then the carbon-intransition metal bond is ionic. But this idea seems to be true only for the most electropositive metals - for magnesium. But already 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 bound 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 concerted 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 remetalization, we obtain a special case of a very general process, which is simply called σ-bond metathesis. Do not confuse only with true metathesis of olefins and acetylenes, which are full-fledged catalytic reactions with their own mechanisms. In this case, we are talking about the mechanism of remetalization or another process in which something similar occurs.

Introduction to work

The relevance of the work. Complexes of porphyrins with metals in high oxidation states can coordinate bases much more efficiently than M2+ complexes and form mixed coordination compounds in which, in the first coordination sphere of the central metal atom, along with the macrocyclic ligand, there are noncyclic acidoligands, and sometimes coordinated molecules. The issues of compatibility of ligands in such complexes are extremely important, since it is in the form of mixed complexes that porphyrins perform their biological functions. In addition, reactions of reversible addition (transfer) of base molecules, characterized by moderately high equilibrium constants, can be successfully used for the separation of mixtures of organic isomers, for quantitative analysis, for the purposes of ecology and medicine. Therefore, studies of the quantitative characteristics and stoichiometry of additional coordination equilibria on metalloporphyrins (MPs) and substitution of simple ligands in them are useful not only from the point of view of theoretical knowledge of the properties of metalloporphyrins as complex compounds, but also for solving the practical problem of searching for receptors and carriers of small molecules or ions. So far, there are practically no systematic studies on complexes of highly charged metal ions.

Goal of the work. This work is devoted to the study of the reactions of mixed porphyrin-containing complexes of highly charged metal cations Zr IV , Hf IV , Mo V and W V with bioactive N-bases: imidazole (Im), pyridine (Py), pyrazine (Pyz), benzimidazole (BzIm), characterization stability and optical properties of molecular complexes, substantiation of stepwise reaction mechanisms.

Scientific novelty. Methods of modified spectrophotometric titration, chemical kinetics, electronic and vibrational absorption and 1 H NMR spectroscopy were used for the first time to obtain thermodynamic characteristics and substantiate the stoichiometric mechanisms of reactions of N-bases with metal porphyrins with a mixed coordination sphere (X) -, O 2-, TPP - tetraphenylporphyrin dianion). It has been established that in the vast majority of cases, the processes of formation of metalloporphyrin-base supramolecules proceed stepwise and include several reversible and slow irreversible elementary reactions of coordination of base molecules and substitution of acidoligands. For each stage of the stepwise reactions, the stoichiometry, equilibrium or rate constants, base orders of slow reactions were determined, and the products were spectrally characterized (UV, visible spectra for intermediate products and UV, visible and IR for final products). Correlation equations have been obtained for the first time, which make it possible to predict the stability of supramolecular complexes with other bases. The equations are used in this work to discuss the detailed mechanism of substitution of OH - in Mo and W complexes by a base molecule. The properties of MR are described, which determine the prospect of using them for the detection, separation, and quantitative analysis of biologically active bases, such as moderately high stability of supramolecular complexes, clear and fast optical response, low sensitivity threshold, and one-second circulation time.

The practical significance of the work. Quantitative results and substantiation of the stoichiometric mechanisms of molecular complex formation reactions are essential for the coordination chemistry of macroheterocyclic ligands. The dissertation work shows that mixed porphyrin-containing complexes exhibit high sensitivity and selectivity with respect to bioactive organic bases, within a few seconds or minutes they give an optical response suitable for the practical detection of reactions with bases - VOCs, components of drugs and food, due to which are recommended for use as components of base sensors in ecology, food industry, medicine and agriculture.

Approbation of work. The results of the work were reported and discussed at:

IX International Conference on Problems of Solvation and Complex Formation in Solutions, Ples, 2004; XII Symposium on Intermolecular Interactions and Conformations of Molecules, Pushchino, 2004; XXV, XXVI and XXIX Scientific Sessions of the Russian Seminar on the Chemistry of Porphyrins and Their Analogues, Ivanovo, 2004 and 2006; VI School-Conference of young scientists of the CIS countries on the chemistry of porphyrins and related compounds, St. Petersburg, 2005; VIII scientific school - conferences on organic chemistry, Kazan, 2005; All-Russian scientific conference "Natural macrocyclic compounds and their synthetic analogues", Syktyvkar, 2007; XVI International Conference on Chemical Thermodynamics in Russia, Suzdal, 2007; XXIII International Chugaev Conference on Coordination Chemistry, Odessa, 2007; International Conference on Porphyrins and Phtalocyanines ISPP-5, 2008; 38th International Conference on Coordination Chemistry, Israel, 2008.

The main substitution reaction in aqueous solutions - the exchange of water molecules (22) - was studied for a large number of metal ions (Fig. 34). The exchange of water molecules in the coordination sphere of a metal ion with the bulk of water molecules present as a solvent proceeds very quickly for most metals, and therefore the rate of such a reaction was studied mainly by the relaxation method. The method consists in disturbing 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. Then measure the rate of equilibrium. If it is possible to change the temperature of the solution within 10 -8 sec, then it is possible to measure the rate of a reaction that requires a time interval greater than 10 -8 sec.

It is also possible to measure the rate of substitution of coordinated water molecules in various metal ions by ligands SO 2-4, S 2 O 3 2- , EDTA, etc. (26). The rate of such a 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 velocity 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 H 2 O molecules or SO 4 2- , S 2 O 3 2- , or EDTA ions.

This observation, and the fact that the rate equation for this process does not include the concentration of the incoming ligand, suggests that these reactions proceed by a mechanism in which the slow step is to break the bond between the metal ion and water. The resulting compound is then likely to rapidly coordinate nearby ligands.

In sec. 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 M 2+ and M + ions; this gives grounds to assume that bond breaking plays an important role in the stage that determines the rate of the entire process. The conclusions obtained in these studies are not conclusive, but they give reason to believe that S N 1 processes are important in substitution reactions of hydrated metal ions.

Probably the most studied complex compounds are cobalt(III) ammines. Their stability, ease of preparation, and slow reactions with them make them especially suitable for kinetic studies. Since the studies of these complexes were carried out exclusively in aqueous solutions, it is first necessary to consider the reactions of these complexes with 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 ligands other than amines is usually considered.

The rate of reactions of type (28) was studied and found to be of the first order with respect to the cobalt complex (X is one of 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 changing the concentration of water molecules on the reaction rate. The rate equations (29) and (30) for an aqueous solution are experimentally indistinguishable, since k is simply equal to k" = k". Therefore, it is impossible to tell from the reaction rate equation whether H 2 O will participate in the step 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 according to 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.

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

Another way of proof is based on the study of the hydrolysis of a series of complexes similar to trance- + . In these complexes, the ethylenediamine molecule is replaced by similar 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. Replacing hydrogen atoms with CH 3 groups increases the volume of the ligand, which makes it difficult for another ligand to attack the metal atom. These steric hindrances 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 that the reaction proceeds according to the S N 1 mechanism.

So, as a result of numerous studies of Co(II) acidoamine complexes, it turned out that the replacement of acid groups by water molecules is a dissociative process in nature. The cobalt atom-ligand bond lengthens to a certain critical value before water molecules begin to enter the complex. In complexes with a charge of 2+ and higher, the breaking 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 acido group (X -) in the cobalt(III) complex with a group other than the H 2 O molecule (31) first proceeds through its substitution by the molecule

solvent - 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 in terms of reactivity with Co(III) amines. It reacts very quickly with amminic complexes of cobalt(III) (about 10 6 times faster than water) according to the type of reaction basic hydrolysis (33).

This reaction was found to be first order with respect to the substituting ligand OH - (34). The overall second order of the reaction and the unusually fast progression of the reaction suggest that the OH ion is an exceptionally effective nucleophilic reagent with respect to Co(III) complexes and that the reaction proceeds via the S N 2 mechanism through the formation of an intermediate.

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 an acid 2+.

Then the reaction proceeds according to the S N 1 mechanism (36) with the formation of a five-coordination 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 rate-determining step involves a base conjugated to the initial acid complex, this mechanism is given the designation S N 1CB.

It is very difficult to determine which of these mechanisms best explains experimental observations. However, there is strong evidence supporting 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 S N 1 mechanism, and there are no convincing arguments why the OH ion should cause the S N 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 unreasonable. Reactions with cobalt(III) compounds in non-aqueous media provide excellent evidence for the formation of five-coordination intermediates provided for 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 than the nucleophilic properties of OH for the rate of the reaction. This reaction of the basic hydrolysis of amminic Co(III) complexes is an illustration of the fact that kinetic data can often be interpreted in more than one way, and In order to exclude this or that possible mechanism, it is necessary to carry out a rather subtle experiment.

At present, substitution reactions of a large number of octahedral compounds have been studied. If we consider their reaction mechanisms, then the dissociative process is most often encountered. 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 where the occurrence of a seven-coordination intermediate has been proven or the effect of an intruding 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 complexes are divided into exchange, redox, isomerization, and coordinated ligands.

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

Xm + mNaY = Ym + mNaX.

The components of the inner sphere of the complexes can also participate in exchange processes involving both the ligands and the complexing agent. To characterize the substitution reactions 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 substitutions:

Z + Y = z + X S N

Z + M"= z + M S E .

According to 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 ), which differ in the transition state with an increased and decreased coordination number.

Assigning the reaction mechanism to associative or dissociative is a difficult experimentally achievable task 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 in complexes, the 1983 Nobel laureate G. Taube (Fig. 44) suggested using the terms "labile" and "inert" depending on the time of the ligand substitution reaction less 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 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 the ligands. According to the ligand field theory:

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

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

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

2. Octahedral complexes 3 d transition metals with free sigma* leavening e g orbitals and a 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. Plano-square and octahedral 4 d and 5d transition metals that do not have electrons per sigma* loosening MO 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 “mutual influence of ligands” model. A special case of the model of mutual influence of ligands is formulated in 1926 by I.I. Chernyaev the concept of trans-influence (Fig. 45) - "the lability of the ligand in the complex depends on the nature of the trans-located ligand" - and propose a series of trans-influence 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 made it possible to substantiate the rules of thumb:

1. Peyronet's rule- under the action of ammonia or amines on tetrachloroplatinate ( II ) potassium is always obtained dichlordiaminplatinum cis-configuration:

2 - + 2NH 3 \u003d cis - + 2Cl -.

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

2- + NH 3 \u003d -

NH 3 \u003d cis -.

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

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

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

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

cis - + 4Thio \u003d 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 replacement 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 trans to thiourea undergo further substitution, which leads to the formation 2+ :

cis - 2+ + 2Thio \u003d 2+ + 2NH 3.

In trans-[ Pt (NH 3 ) 2 (Thio ) 2 ] 2+ two ammonia molecules with a small trans effect 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 when studying ligand substitution reactions in square-planar platinum complexes ( II ). Subsequently, it was shown that the trans effect of ligands also manifests itself 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 trance influence is kinetic effect- the greater the trans-influence of this ligand, the faster the replacement of another ligand, which is in relation to it in the trans-position.

Along with the kinetic effect of trans-influence, in the middle XX century A.A. Grinberg and Yu.N. Kukushkin established the dependence of the trans effect of the ligand L from the ligand in the cis position to L . Thus, the study of the rate of 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 \u003d 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 or two ammonia molecules in the cis-position to the chloride ligand being replaced leads to a successive increase in the reaction rate. This kinetic effect is called cis influence. At present, both kinetic effects of the influence of the nature of ligands on the rate of ligand substitution reactions (trans- and cis-effects) are combined in a common concept mutual influence of ligands.

The theoretical substantiation of the effect of the mutual influence of ligands is closely connected with 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 characteristic of complexes whose central metal ion has a high polarizability.

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

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

In 1943 A.A. Greenberg suggested that the trans activity of ligands is related to their reducing properties. The shift of the electron density from the trans-active ligand to the metal reduces the effective charge of the metal ion, which leads to a weakening of the chemical bond with the 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 for trans-located ligands. Coordination to the metal ion of the attacking ligand Z leads to the formation of a five-coordinate trigonal-bipyramidal intermediate, followed by rapid cleavage of the outgoing ligand X. The formation of such an intermediate is facilitated bypi-dative ligand-metal ligand interaction Y , which reduces the electron density of the metal and reduces the activation energy of the transition state with subsequent rapid substitution of the X ligand.

Along with 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 between the metal and the outgoing ligand Y .

Thus, the position of ligands in the trans activity series is determined by the combined 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.

One of the most important steps in metal complex catalysis, the interaction of the Y substrate with the complex, proceeds via three mechanisms:

a) Replacement of the ligand with a solvent. Usually such a stage is depicted as the dissociation of the complex

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

b) Attachment of a new ligand along a free coordinate with the formation of an associate, followed by dissociation of the substituted 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-way equation

Where k S And k Y are the rate constants of the processes occurring in 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 - cleavage of Cl - , addition of Y and elimination of the H 2 O molecule.

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

H2O~NH3

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

Reactions of coordinated ligands

Reactions of electrophilic substitution (S E) of hydrogen by a metal in the coordination sphere of the metal and their reverse processes

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 bond M-X

In the case of X=R (an organometallic complex), metal-coordinated molecules are also introduced at the M-R bond (L–CO, RNC, C 2 H 2 , C 2 H 4 , N 2 , CO 2 , O 2 , etc.). Insertion reactions are the result of an intramolecular attack by nucleophile X on a - or -coordinated molecule. Reverse reactions - - and -elimination reactions

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 the solvent, which is easily replaced by the substrate, is an important factor affecting the reactivity of metal complexes. For example, bis--allyl complexes of Ni are good precursors of catalytically active species, since due to the easy reductive elimination of bis-allyl, a complex with a solvent, the so-called. bare nickel. The role of free seats is illustrated by the following example:

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

1. Reactions of organometallic compounds

As intermediates of catalytic reactions, there are both classical organometallic compounds having M-C, M=C and MC bonds, as well as non-classical compounds in which the organic ligand is coordinated according to  2 ,  3 ,  4 ,  5 and  6 -type, or is an element of electron-deficient structures - bridging CH 3 and C 6 H 6 groups, non-classical 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

AdEAddition-elimination

AdE(C) Attachment to C atom in sp 2 hybridization

AdE(M) Oxidative addition to metal

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

It is possible that an oxidizing agent may be involved in this step.

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:

M-C bond hydrogenolysis

and homolysis of the M-C bond

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