Complex compounds of d-elements. Chemistry of aqueous solutions: hydrolysis, polymerization, polycondensation

Elementary stages involving coordination and metal organic compounds in solutions and on the surface of metals and oxides

Elementary stages organic reactions, catalyzed by acids, bases, nucleophilic catalysts, metal complexes, solid metals and their compounds in gas-phase or liquid-phase heterogeneous and homogeneous processes, are reactions of the formation and transformation of various organic and organometallic intermediates, as well as metal complexes. Organic intermediate compounds include carbenium ions R + , carbonium RH 2 + , carbo-anions R-, anion- and radical cations, radicals and biradicals R·, R:, as well as molecular complexes of organic donor and acceptor molecules (DA), which are called also by complexes with charge transfer. In homogeneous and heterogeneous catalysis by metal complexes (metal complex catalysis) of organic reactions, intermediates are complex (coordination) compounds with organic and inorganic ligands, organometallic compounds with M-S connection, which in most cases are coordination compounds. A similar situation occurs in the case of “two-dimensional” chemistry on the surface of solid metal catalysts. Let us consider the main types of reactions of metal complexes and organometallic compounds.

Elementary stages involving metal complexes

Reactions of metal complexes can be divided into three groups:

a) electron transfer reactions;

b) ligand substitution reactions;

c) reactions of coordinated ligands.

Electron transfer reactions

Two mechanisms are implemented in electron transfer reactions - the outer-sphere mechanism (without changes in the coordination spheres of the donor and acceptor) and the bridging (inner-sphere) mechanism, leading to changes in the coordination sphere of the metal.

Let us consider the outer-sphere mechanism using the example of octahedral complexes of transition metals. In the case of symmetric reactions ( G 0 = 0)

rate constants vary in a very wide range of values ​​- from 10-12 to 10 5 l mol-1 sec-1, depending on the electronic configuration of the ion and the degree of its restructuring during the process. In these reactions, the principle of least movement is very clearly manifested - the least change in the valence shell of the reaction participants.

In the electron transfer reaction (1) (Co * is an isotope of the Co atom)

(symmetric reaction), Co 2+ (d 7) goes into Co 3+ (d 6). The electronic configuration (valence shell) does not change during this transfer

6 electrons at the triply degenerate bonding level remain unchanged (), and from the antibonding level e g level one electron is removed.
Second order rate constant for reaction (1) k 1 = 1.1 lmol-1 sec-1. Since Phen (phenanthroline) is a strong ligand, the maximum number is 7 d-electrons are paired (spin-paired state). In the case of a weak ligand NH 3 the situation changes radically. Co(NH 3) n 2+ (n = 4, 5, 6) is in a spin-unpaired (high-spin) state.

The stronger complex Co(NH 3) 6 3+ (stronger than Co(NH 3) 6 2+ ~ 10 30 times) is in a spin-paired state, like the complex with Phen. In this regard, in the process of electron transfer, the valence shell should be strongly reconstructed and, as a result, k= 10-9 lmol-1 sec-1. The conversion rate of Co 2+ to Co 3+, equal to 50%, is achieved in the case of the Phen ligand in 1 second, and in the case of NH 3 ~ in 30 years. It is obvious that a stage with such a rate (formally elementary) can be excluded from the set of elementary stages when analyzing reaction mechanisms.

Magnitude G for the electron transfer reaction during the formation of a collision complex, according to Marcus theory, includes two components and

The first term is the energy of reorganization M-L connections inside the complex (length and bond strength when the valence state changes). The quantity includes the energy of reconstruction of the outer solvation shell during the change coordinates M-L and charge of the complex. The smaller the change in the electronic environment and the smaller the change length M-L, the lower, the larger the ligands, the lower and, as a result, the higher the rate of electron transfer. The value for the general case can be calculated using the Marcus equation

Where. At = 0 .

In the case of the intrasphere mechanism, the process of electron transfer is facilitated, since one of the ligands of the first complex forms a bridging complex with the second complex, displacing one of the ligands from it

The rate constants of such a process are 8 orders of magnitude higher than the constants for the reduction of Cr(NH 3) 6 3+. In such reactions, the reducing agent must be a labile complex, and the ligand in the oxidizing agent must be capable of forming bridges (Cl-, Br-, I-, N 3 -, NCS-, bipy).

Ligand substitution reactions

One of the most important stages in metal complex catalysis, the interaction of substrate Y with the complex, occurs through 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 ligand L with solvent S, which is then easily replaced by a substrate molecule Y

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

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

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

Where k S And k Y- rate constants of processes occurring in reactions (5) (with a solvent) and (6) with ligand Y. For example,

The last stage of the second route is the sum of three fast elementary stages - the elimination of Cl-, the addition of Y and the elimination of the H 2 O molecule.

In flat square complexes of transition metals, a trans effect is observed, formulated by I.I. Chernyaev - the influence 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< Cl- ~ Br- < I- ~ NO 2 - ~ C 6 H 5 - < CH 3 - <
< PR 3 ~ AsR 3 ~ H- < олефин ~ CO ~ CN-.

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

Reactions of coordinated ligands

§ Reactions of electrophilic substitution (SE) of hydrogen with a metal in the coordination sphere of the metal and their reverse processes

SH - H 2 O, ROH, RNH 2, RSH, ArH, RCCH.

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

§ Reactions of introduction of L through the M-X connection

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

§ Reactions of oxidative addition and reductive elimination

M 2 (C 2 H 2) M 2 4+ (C 2 H 2) 4-

Apparently, in these reactions there is always preliminary coordination of the added molecule, but this cannot always be detected. In this regard, the presence of a free site in the coordination sphere or a site associated with a solvent, which is easily replaced by the substrate, is 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 the bis-allyl, a complex with the solvent appears, 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

Reactions of organometallic compounds

As intermediates of catalytic reactions, they are found as classical organometallic compounds having M-C communications, M=C and MC, and non-classical compounds, in which the organic ligand is coordinated according to the 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 scientific mechanisms for classical organometallic compounds, we note several mechanisms. Thus, 5 mechanisms of electrophilic substitution of the metal atom at the M-C bond have been established.

electrophilic substitution with nucleophilic assistance

AdE Addition-elimination

AdE(C) Addition to the C atom in sp 2 hybridization

AdE(M) Oxidative addition to metal

Nucleophilic substitution at the carbon atom in demetalation reactions of organometallic compounds occurs as an oxidation-reduction process:

Possible participation of an oxidizing agent in this stage

Such an oxidizing agent can be CuCl 2, p-benzoquinone, NO 3 - and other compounds. 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 that applies to all reactions of complex and organometallic compounds and is associated with the principle of least motion is Tolman's 16-18 electron shell rule (Section 2).

Coordination and organometallic compoundson a surface

According to modern concepts, complexes and organometallic compounds similar to compounds in solutions are formed on the surface of metals. For surface chemistry, the participation of several surface atoms in the formation of such compounds and, of course, the absence of charged particles is essential.

Surface groups can be any atoms (H, O, N, C), groups of atoms (OH, OR, NH, NH 2, CH, CH 2, CH 3, R), coordinated molecules CO, N 2, CO 2, C 2H4, C6H6. For example, during the adsorption of CO on a metal surface, the following structures were found:

The C 2 H 4 molecule on the metal surface forms -complexes with one center and di-connected ethylene bridges M-CH 2 CH 2 -M, i.e. essentially metal cycles

On the surface of Rh, for example, during the adsorption of ethylene, the following processes of ethylene conversion occur as the temperature increases:

Reactions of surface intermediates include the stages of oxidative addition, reductive elimination, insertion, - and -elimination, M-C hydrogenolysis and C-C connections and other reactions of the organometallic type, but without the appearance of free ions. The tables show the mechanisms and intermediates of surface transformations of hydrocarbons on metals.

Table 3.1. Catalytic reactions involving the cleavage of a C-C bond.

Designations:

Alkyl, metallacycle;

Carbene, allyl;

Carbin, vinyl.

Table 3.2. Catalytic reactions involving S-S education communications.

Designations: see table. 3.1.

The formation of all of the above organometallic compounds on the surface of metals has been confirmed by physical methods.

Questions for self-control

1) How does the rule of the smallest change in the valence shell of a metal manifest itself during electron transfer reactions?

2) Why do coordination vacancies contribute to effective interaction with the substrate?

3) List the main types of reactions of coordinated ligands.

4) Give the mechanisms of electrophilic substitution in the reactions of organometallic compounds with NX.

5) Give examples of surface organometallic compounds.

6) Give examples of the participation of metal carbene surface complexes in the transformations of hydrocarbons.

Literature for in-depth study

1. Temkin O.N., Kinetics of catalytic reactions in solutions of metal complexes, M., MITHT, 1980, Part III.

2. Collman J., Higedas L., Norton J., Finke R., Organometallic chemistry of consumer metals, M., Mir, 1989, vol. I, vol. II.

3. Moiseev I.I., -Complexes in the oxidation of olefins, M., Nauka, 1970.

4. Temkin O.N., Shestakov G.K., Treger Yu.A., Acetylene: Chemistry. Mechanisms of reactions. Technology. M., Chemistry, 1991, 416 pp., section 1.

5. Henrici-Olivet G., Olive S., Coordination and catalysis, M., Mir, 1980, 421 p.

6. Krylov O.V., Matyshak V.A., Intermediate compounds in heterogeneous catalysis, M., Nauka, 1996.

7. Zaera F., An Organometallic Guide to the Chemistry of Hydrocarbon Moities on Transition Metal Surfaces., Chem. Rev., 1995, 95, 2651 - 2693.

8. Bent B.E., Mimicking Aspects of Heterogeneous Catalysis: Generating, Isolating, and Reacting Proposed Surface Intermediates on Single Crystals in Vacuum, Chem. Rev., 1996, 96, 1361 - 1390.

Conditionally chemical reactions complexes are divided into exchange, redox, isomerization and coordinated ligands.

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

X m + mNaY = Y m + mNaX.

Components of the internal sphere of complexes can also participate in metabolic processes involving both ligands and the complexing agent. To characterize substitution reactions of ligands or the central metal ion, use the designations 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 ), differing in the transition state with an increased and decreased coordination number.

Classifying a reaction mechanism as 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, 1983 Nobel laureate G. Taube (Fig. 44) proposed using the terms “labile” and “inert” depending on the time of the ligand substitution reaction, 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 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. In accordance with ligand field theory:

1. Octahedral complexes 3 d transition metals with 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 energy of stabilization by the crystal field of the complex, the greater its lability.

2. Octahedral complexes 3 d transition metals with free sigma* loosening 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 5 d transition metals that do not have 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 “mutual influence of ligands” model. A special case of the model of mutual influence of ligands is that formulated in 1926 by I.I. Chernyaev's 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 number of trans-influences of the 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 us to justify the rules of thumb:

1. Peyrone's rule- due to the action of ammonia or amines on tetrachloroplatinate ( II ) potassium is always obtained dichlorodiamineplatinum cis-configuration:

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

Since the reaction proceeds in two stages and the chloride ligand has a large trans influence, 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 - upon the action of hydrochloric acid on platinum tetrammine chloride ( II ) or similar compounds is obtained dichlorodi-ammineplatinum trans configuration:

[ 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, 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 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 = 2+ + 2Cl - + 2NH 3 .

The different nature of the reaction products is associated with the high trans influence 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 in 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 little trans influence are located in a 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-influence 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 structure. True, the series of trans-influence of ligands for different metals are somewhat different.

It should be noted that trans influence is kinetic effect- the greater the trans influence of a given ligand, the faster it is replaced by another ligand that is in a trans position relative to it.

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-influence of the ligand L from the ligand located in 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 = 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 replaced chloride ligand leads to a consistent increase in the reaction rate. This kinetic effect is called cis influence. Currently, both kinetic effects of the influence of the nature of the ligands on the rate of ligand substitution reactions (trans- and cis-effect) are combined in a general concept mutual influence of ligands.

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

1. The trans effect is typical for complexes whose central metal ion is highly polarizable.

2. The trans activity of ligands is determined by the energy of mutual polarization of the ligand and the metal ion. For a given metal ion, the trans influence 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, such as halide ions.

In 1943 A.A. Greenberg hypothesized that the trans activity of ligands is related to their reducing properties. The shift in electron density from the trans-located 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 trans influence is associated with the high trans activity of ligands based on unsaturated organic molecules, similar to ethylene in [ Pt(C2H4)Cl3 ] - . 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 elimination of the leaving 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 replacement of ligand X.

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-influence of such ligands is determined by the donor-acceptor interaction of 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 the ligands in the series of trans-activity 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-influence, whereaspi-donor ones weaken. Which of these components of the ligand-metal interaction predominates in the trans-influence is judged on the basis of quantum chemical calculations electronic structure transition state of the reaction.

Complex connections. Their structure is based on the coordination theory of A. Werner. Complex ion, its charge. Cationic, anionic, neutral complexes. Nomenclature, examples.

Ligand substitution reactions. Instability constant of a complex ion, stability constant.

To instability is the ratio of the products of the concentration of decayed ions by the undecayed amount.

K set = 1/ K nest (reciprocal)

Secondary dissociation - disintegration of the inner sphere of the complex into its constituent components.

43.Competition for a ligand or for a complexing agent: isolated and combined equilibria of ligand substitution. General constant for the combined equilibrium of ligand substitution.

As a result of competition, the proton destroys a fairly strong complex, forming a weakly dissociating substance - water.

Cl + NiS0 4 +4NH 3 ^ S0 4 +AgCl I

This is already an example of ligand competition for a complexing agent, with the formation of a more stable complex (K H + = 9.3-1(G 8 ; K H [M(W 3) 6 ] 2+ = 1.9-10 -9) and a sparingly soluble compound AgCl - K s = 1.8 10" 10

Ideas about the structure of metalloenzymes and other biocomplex compounds (hemoglobin, cytochromes, cobalamins). Physicochemical principles of oxygen transport by hemoglobin

Cobalamins. Vitamins B 12 called a group of cobalt-containing biological active substances called cobalamins. These actually include cyanocobalamin, hydroxycobalamin and two coenzyme forms of vitamin B 12: methylcobalamin and 5-deoxyadenosylcobalamin.

Sometimes more in the narrow sense vitamin B 12 is called cyanocobalamin, since it is in this form that the human body receives the bulk of vitamin B 12, without losing sight of the fact that it is not synonymous with B 12, and several other compounds also have B 12 vitamin activity. Vitamin B 12 is also called Castle's extrinsic factor.

B 12 has the most complex chemical structure compared to other vitamins, the basis of which is the corrine ring. Corrin is in many ways similar to porphyrin (a complex chemical structure that is part of heme, chlorophyll and cytochromes), but differs from porphyrin in that the two pyrrole rings in corrin are connected directly to each other, and not by a methylene bridge. A cobalt ion is located in the center of the corrin structure. Cobalt forms four coordination bonds with nitrogen atoms. Another coordination bond connects cobalt with a dimethylbenzimidazole nucleotide. The last, sixth coordination bond of cobalt remains free: it is through this bond that a cyano group, a hydroxyl group, a methyl or 5"-deoxyadenosyl residue is added to form four variants of vitamin B 12, respectively. The covalent carbon-cobalt bond in the structure of cyanocobalamin is the only one known in life nature is an example of a transition metal-carbon covalent bond.

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

• a coordinatively unsaturated complex binds a new ligand
• in an already completed coordination sphere, one ligand is replaced by another.

We have already become familiar with the first method when we discussed coordination unsaturation and the 18-electron rule. We'll deal with 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, the electron count does not change during substitution. Substitution of one type of ligand for another is quite possible and often occurs in reality. Let us only pay attention to the correct handling of charges when changing the L-ligand to the X-ligand and vice versa. If we forget about this, then the oxidation state of the metal will change, and the replacement of ligands is not an oxidation-reduction process (if you find or come up with a contrary example, let me know - it will be automatically credited right away, if I cannot prove that you were mistaken, even in In this case, I guarantee a positive contribution to karma).

Substitution involving hapto ligands

With more complex ligands there are no more difficulties - you just need to remember a fairly obvious rule: the number of ligand sites (that is, the total number of ligands or X- or L-type ligand centers) is maintained. This follows directly from the conservation of electron counting. Here are self-evident examples.

Let's pay attention to 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 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 “just salts”; any derivatives are coordination compounds, to which all considerations about electron counting, d-configuration, coordination saturation, etc. apply. Iron dichloride, as we are used to writing it, would turn out to be a Fe(2+) complex of type MX 2 with configuration d 6 and number of electrons 10. Not enough! Fine? After all, we have already figured out that ligands can be implicit. To make the reaction we need a solvent, and for such reactions it is most likely THF. The dissolution of a crystalline iron salt in THF occurs precisely because the donor solvent occupies free spaces, and the energy of this process compensates for the destruction crystal lattice. We would not be able to dissolve this “salt” in a solvent that does not provide the metal solvation services due to Lewis basicity. In this case, and in a million similar ones, 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 molecules of the donor solvent with formation of a charged complex FeL 6 2+. In this case it is not so important. Either way, we can safely assume that we have an 18-electron complex on both the left and 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 occurred in two stages: the old substituent first left, leaving a vacant orbital on the carbon atom, which was then occupied by a new substituent with a pair of electrons. The second mechanism assumed that leaving and coming were 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 detach the old one. It immediately becomes clear that this third option is hardly possible if the complex already has 18 electrons and is coordination 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 occurs 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 occurs as an abstraction-addition (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 unprofitable and every time in case of suspicion of such a path, very significant evidence is required. In most of these stories, the researchers eventually concluded that they had overlooked or missed 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 must take its place, for example:

If we want to introduce a hapto-ligand occupying several sites into the coordination sphere, then we must first vacate them all. As a rule, this occurs only under fairly severe conditions, for example, in order to replace three carbonyls in chromium carbonyl with η 6 -benzene, the mixture is heated under pressure for many hours, releasing the released carbon monoxide from time to time. Although the diagram 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 at a time, and benzene entering the sphere, gradually increasing hapticity, through the stages minus CO - digapto - 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 replacement of the ligand most likely occurs as an addition-elimination (for those who like deep-sounding 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 are 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 throw out the extra ligand, returning to their favorite configuration. Sometimes the spatial factor also interferes with the matter; the existing ligands are large and the additional one feels like a bus passenger at rush hour. It’s easier to get off and take a walk than to suffer like this. However, you can push out another passenger, let him take a walk, and we will 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 variety, let's replace the X-ligand with an L-ligand, and we won't get confused about oxidation states and charges. Once again: upon substitution, the oxidation state does not change, and if the X-ligand has left, then the loss must be compensated for by the charge on the metal. If we forget about this, then the oxidation number would decrease by 1, but this is incorrect.

And one more strange thing. 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 the pyridine nitrogen (for example, upon protonation or 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 accustomed to the strict and unambiguous system of drawing structures in organic chemistry, but you will have to get used to it, it is not so difficult.

But 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

We could not talk about the mechanisms of ligand substitution at all if 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 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 don’t want such simple, “unscientific” words, use English word spectator in local transcription, spectator, ligand-spectator, but, I beg you, not spectator - this is unbearable!). And some directly participate in the reaction, turning into reaction products. Such ligands are called actors (not actors!), that is, active ones. It is quite clear that 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 spectator ligands 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 ligand actors and in the required quantities can participate in the right process. If there are more available coordination sites than necessary, extra ligand actors may 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, they ensure the solubility of complexes, stabilize the correct valence state of the metal, especially if it is not quite ordinary, help individual stages, provide stereoselectivity, etc. We won’t 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 should be tightly bound and not prone to dissociation and replacement by other ligands. Such ligands are usually called coordinationally stable . Or simply stable, if it is clear from the context that we are talking about the strength of the bonds of the ligands, and not about their own thermodynamic stability, which does not concern us at all.

And ligands that easily and willingly enter and leave, 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 leave the warm and cozy sphere outside, nothing good will await it (at the cost of the output will be precisely the energy of anti-aromatic destabilization).

Cyclobutadiene and its derivatives are the most famous examples anti-aromaticity. 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 maximally weakening the conjugation of double bonds (this is otherwise called the Jahn-Teller effect of the 2nd 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 complexes, cyclobutadiene and substituted cyclobutadienes are excellent tetrahapto ligands, and the geometry of such ligands is exactly a square, with identical bond lengths. How and why this happens is a separate story, and not nearly as obvious as it is often made out to be.

Coordination labile ligands

You need to understand that there is no reinforced concrete fence with barbed wire and security towers between the areas of labile and stable ligands. Firstly, it depends on the metal, and LMKO works well in this context. For example, late transition metals prefer soft ligands, while early transition metals prefer hard ones. Let's say, iodide holds very tightly to the d 8 atoms of palladium or platinum, but rarely enters the coordination sphere of titanium or zirconium in the d 0 configuration at all. But in many metal complexes with less pronounced features, iodide manifests itself as a completely labile ligand, easily giving way to others.

Other things being equal:

• L-ligands are usually 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 completely neglected and structures without them are drawn with a formally unsaturated coordination sphere;
• Dihapto ligands, for example alkenes and alkynes, behave like typical L-ligands: they are usually quite labile;
• ligands with greater hapticity are rarely labile, but if a polyhapto ligand can change the mode of binding to 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, and chelates with smaller or a large number atoms of the cycle are labile, at least at one center (the chelate ring opens and the ligand remains hanging as a simple one). This is how acetate behaves, for example;

Coordinatively stable ligands

Let's repeat it all again, only on the other side

In the coordination sphere of metals, the following are generally preserved (coordinationally stable):

• 5- and 6-membered chelators;
• polyhapto-ligands: in order to knock cyclopentadienyls or benzene (arenes) out of the coordination sphere, you have to use all sorts of special techniques - they just don’t come out, often withstanding even prolonged heating;
• ligands bound to a metal with a high proportion of π-donor effect (back-donation);
• soft ligands for late transition metals;
• “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 absolutely stable ones (no chelators or polyhapto-ligands). Then, in reactions, the ligands will change, relatively speaking, in order of relative lability. The least labile and the last to remain. This trick occurs, 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 after another, to actor ligands. 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 fairly 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 the reactions of transition metal complexes - the concept of ligand control. We'll discuss it later.

Remetallation

When replacing some ligands with others, it is important not to overdo 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 reactant into 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 saturation of the coordination sphere and the impossibility of introducing into it other ligands necessary for the expected process. This problem especially often arises 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 the alkali metal cation, which determines the high ionicity of the bond, less electropositive metals and metalloids (zinc, tin, boron, silicon, etc.) are used, forming covalent bonds with the nucleophilic part . Reactions of such derivatives with transition metal derivatives produce ligand substitution products, in principle just as if the nucleophile were in anionic form, but due to reduced nucleophilicity with less 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. This name, therefore, contains an element of unpleasant schizophrenia - we seemed to have already agreed that we would look at all reactions from the point of view of a transition metal, but suddenly we lost it again and look at this reaction and only this reaction from the point of view of a nucleophile. You will have to be patient, this is how the terminology has developed and 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 halides of various metals and metalloids is one of the main methods for the synthesis of all organometallic compounds, primarily intransition ones, and the reaction that we are now considering in chemistry of coordination compounds of transition metals is simply a generalization of the ancient method of organometallic chemistry from which it all grew.

How does transmetallation occur?

Remetallation is both similar to conventional substitution and not similar. It looks like - if we consider a non-transition organometallic reagent to be simply a carbanion with a counterion, then the carbon-non-transition metal bond is ionic. But this idea seems to be true only for the most electropositive metals - 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 bond to each other in a different order. Most likely, all this occurs simultaneously in a four-member transition state, and the reaction itself has a concerted character, like so many other reactions of transition metals. The abundance of electrons and orbitals for literally all tastes and all types of symmetries makes transition metals capable of simultaneously maintaining bonds in transition states with several atoms.

In the case of remetallization, we obtain a special case of very general process, which is simply called σ-bond metathesis. Do not confuse them 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 transmetallation or another process in which something similar occurs.

Chapter 17. Complex connections

17.1. Basic definitions

In this chapter, you will become familiar with a special group of complex substances called comprehensive(or coordination) connections.

Currently, a strict definition of the concept " complex particle" No. The following definition is usually used.

For example, a hydrated copper ion 2 is a complex particle, since it actually exists in solutions and some crystalline hydrates, it is formed from Cu 2 ions and H 2 O molecules, water molecules are real molecules, and Cu 2 ions exist in crystals of many copper compounds. On the contrary, the SO 4 2 ion is not a complex particle, since, although O 2 ions occur in crystals, the S 6 ion does not exist in chemical systems.

Examples of other complex particles: 2, 3, , 2.

At the same time, NH 4 and H 3 O ions are classified as complex particles, although H ions do not exist in chemical systems.

Sometimes complex chemical particles are called complex particles, all or part of the bonds in which are formed according to the donor-acceptor mechanism. In most complex particles this is the case, but, for example, in potassium alum SO 4 in complex particle 3, the bond between the Al and O atoms is actually formed according to the donor-acceptor mechanism, and in the complex particle there is only an electrostatic (ion-dipole) interaction. This is confirmed by the existence in iron-ammonium alum of a complex particle similar in structure, in which only ion-dipole interaction is possible between water molecules and the NH 4 ion.

Based on their charge, complex particles can be cations, anions, or neutral molecules. Complex compounds containing such particles can belong to different classes of chemical substances (acids, bases, salts). Examples: (H 3 O) is an acid, OH is a base, NH 4 Cl and K 3 are salts.

Typically the complexing agent is an atom of the element that forms the metal, but it can also be an atom of oxygen, nitrogen, sulfur, iodine, and other elements that form nonmetals. The oxidation state of the complexing agent can be positive, negative or zero; when a complex compound is formed from simpler substances, it does not change.

Ligands can be particles that, before the formation of a complex compound, were molecules (H 2 O, CO, NH 3, etc.), anions (OH, Cl, PO 4 3, etc.), as well as a hydrogen cation. Distinguish unidentate or monodentate ligands (connected to the central atom through one of their atoms, that is, by one -bond), bidentate(connected to the central atom through two of their atoms, that is, by two -bonds), tridentate etc.

If the ligands are unidentate, then the coordination number is equal to the number of such ligands.

The CN depends on the electronic structure of the central atom, its oxidation state, the size of the central atom and ligands, the conditions for the formation of the complex compound, temperature and other factors. CN can take values ​​from 2 to 12. Most often it is six, somewhat less often – four.

There are complex particles with several central atoms.

Two types of structural formulas of complex particles are used: indicating the formal charge of the central atom and ligands, or indicating the formal charge of the entire complex particle. Examples:

To characterize the shape of a complex particle, the concept of a coordination polyhedron (polyhedron) is used.

Coordination polyhedra also include a square (CN = 4), a triangle (CN = 3) and a dumbbell (CN = 2), although these figures are not polyhedra. Examples of coordination polyhedra and complex particles with corresponding shapes for the most common CN values ​​are shown in Fig. 1.

17.2. Classification of complex compounds

How chemical substances complex compounds are divided into ionic compounds (they are sometimes called ionic) and molecular ( nonionic) connections. Ionic complex compounds contain charged complex particles - ions - and are acids, bases or salts (see § 1). Molecular complex compounds consist of uncharged complex particles (molecules), for example: or - classifying them into any main class of chemical substances is difficult.

The complex particles included in complex compounds are quite diverse. Therefore, several classification features are used to classify them: the number of central atoms, the type of ligand, the coordination number and others.

According to the number of central atoms complex particles are divided into single-core And multi-core. The central atoms of multinuclear complex particles can be connected to each other either directly or through ligands. In both cases, the central atoms with ligands form a single internal sphere of the complex compound:

Based on the type of ligands, complex particles are divided into

1) Aqua complexes, that is, complex particles in which water molecules are present as ligands. Cationic aqua complexes m are more or less stable, anionic aqua complexes are unstable. All crystal hydrates belong to compounds containing aqua complexes, for example:

Mg(ClO 4) 2. 6H 2 O is actually (ClO 4) 2;
BeSO 4. 4H 2 O is actually SO 4;
Zn(BrO 3) 2. 6H 2 O is actually (BrO 3) 2;
CuSO4. 5H 2 O is actually SO 4. H2O.

2) Hydroxo complexes, that is, complex particles in which hydroxyl groups are present as ligands, which were hydroxide ions before entering the composition of the complex particle, for example: 2, 3, .

Hydroxo complexes are formed from aqua complexes that exhibit the properties of cationic acids:

2 + 4OH = 2 + 4H 2 O

3) Ammonia, that is, complex particles in which NH 3 groups are present as ligands (before the formation of a complex particle - ammonia molecules), for example: 2, , 3.

Ammonia can also be obtained from aquatic complexes, for example:

2 + 4NH 3 = 2 + 4 H 2 O

The color of the solution in this case changes from blue to ultramarine.

4) Acid complexes, that is, complex particles in which acid residues of both oxygen-free and oxygen-containing acids are present as ligands (before the formation of a complex particle - anions, for example: Cl, Br, I, CN, S 2, NO 2, S 2 O 3 2 , CO 3 2 , C 2 O 4 2 , etc.).

Examples of the formation of acid complexes:

Hg 2 + 4I = 2
AgBr + 2S 2 O 3 2 = 3 + Br

The latter reaction is used in photography to remove unreacted silver bromide from photographic materials.
(When developing photographic film and photographic paper, the unexposed part of the silver bromide contained in the photographic emulsion is not reduced by the developer. To remove it, this reaction is used (the process is called “fixing”, since the unremoved silver bromide gradually decomposes in the light, destroying the image)

5) Complexes in which hydrogen atoms are the ligands are divided into two completely different groups: hydride complexes and complexes included in the composition onium connections.

During the formation of hydride complexes – , , – the central atom is an electron acceptor, and the donor is the hydride ion. The oxidation state of hydrogen atoms in these complexes is –1.

In onium complexes, the central atom is an electron donor, and the acceptor is a hydrogen atom in the +1 oxidation state. Examples: H 3 O or – oxonium ion, NH 4 or – ammonium ion. In addition, there are substituted derivatives of such ions: – tetramethylammonium ion, – tetraphenylarsonium ion, – diethyloxonium ion, etc.

6) Carbonyl complexes - complexes in which CO groups are present as ligands (before the formation of the complex - molecules of carbon monoxide), for example: , , etc.

7) Anion halogenates complexes – complexes of type .

Based on the type of ligands, other classes of complex particles are also distinguished. In addition, there are complex particles with different types of ligands; The simplest example is aqua-hydroxo complex.

17.3. Basics of complex compound nomenclature

The formula of a complex compound is compiled in the same way as the formula of any ionic substance: the formula of the cation is written in the first place, and the formula of the anion in the second place.

The formula of a complex particle is written in square brackets in the following sequence: the symbol of the complex-forming element is placed first, then the formulas of the ligands that were cations before the formation of the complex, then the formulas of the ligands that were neutral molecules before the formation of the complex, and after them the formulas of the ligands, which were anions before the formation of the complex.

The name of a complex compound is constructed in the same way as the name of any salt or base (complex acids are called hydrogen or oxonium salts). The name of the compound includes the name of the cation and the name of the anion.

The name of the complex particle includes the name of the complexing agent and the names of the ligands (the name is written in accordance with the formula, but from right to left. For complexing agents, the Russian names of the elements are used in cations, and Latin ones in anions.

Names of the most common ligands:

H 2 O – aqua	Cl – chloro	SO 4 2 – sulfato	OH – hydroxo
CO – carbonyl	Br – bromo	CO 3 2 – carbonato	H – hydrido
NH 3 – ammine	NO 2 – nitro	CN – cyano	NO – nitroso
NO – nitrosyl	O 2 – oxo	NCS – thiocyanato	H+I – hydro

Examples of names of complex cations:

Examples of names of complex anions:

2 – tetrahydroxozincate ion
3 – di(thiosulfato)argentate(I) ion
3 – hexacyanochromate(III) ion
– tetrahydroxodiaquaaluminate ion
– tetranitrodiammine cobaltate(III) ion
3 – pentacyanoaquaferrate(II) ion

Examples of names of neutral complex particles:

More detailed nomenclature rules are given in reference books and special manuals.

17.4. Chemical bonds in complex compounds and their structure

In crystalline complex compounds with charged complexes, the bond between the complex and the outer-sphere ions is ionic, the bonds between the remaining particles of the outer sphere are intermolecular (including hydrogen). In molecular complex compounds, the connection between the complexes is intermolecular.

In most complex particles, the bonds between the central atom and the ligands are covalent. All of them or part of them are formed according to the donor-acceptor mechanism (as a consequence - with a change in formal charges). In the least stable complexes (for example, in aqua complexes of alkali and alkaline earth elements, as well as ammonium), the ligands are held by electrostatic attraction. Bonding in complex particles is often called donor-acceptor or coordination bonding.

Let us consider its formation using the example of iron(II) aquacation. This ion is formed by the reaction:

FeCl 2cr + 6H 2 O = 2 + 2Cl

Electronic formula of the iron atom is 1 s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6. Let's draw up a diagram of the valence sublevels of this atom:

When a doubly charged ion is formed, the iron atom loses two 4 s-electron:

The iron ion accepts six electron pairs of oxygen atoms of six water molecules into free valence orbitals:

A complex cation is formed, the chemical structure of which can be expressed by one of the following formulas:

The spatial structure of this particle is expressed by one of the spatial formulas:

The shape of the coordination polyhedron is octahedron. All Fe-O bonds are the same. Supposed sp 3 d 2 - AO hybridization of the iron atom. Magnetic properties complex indicate the presence of unpaired electrons.

If FeCl 2 is dissolved in a solution containing cyanide ions, then the reaction occurs

FeCl 2cr + 6CN = 4 + 2Cl.

The same complex is obtained by adding a solution of potassium cyanide KCN to a solution of FeCl 2:

2 + 6CN = 4 + 6H 2 O.

This suggests that the cyanide complex is stronger than the aqua complex. In addition, the magnetic properties of the cyanide complex indicate the absence of unpaired electrons in the iron atom. All this is due to the slightly different electronic structure of this complex:

“Stronger” CN ligands form stronger bonds with the iron atom, the gain in energy is enough to “break” Hund’s rule and release 3 d-orbitals for lone pairs of ligands. The spatial structure of the cyanide complex is the same as that of the aqua complex, but the type of hybridization is different - d 2 sp 3 .

The “strength” of the ligand depends primarily on the electron density of the cloud of lone pairs of electrons, that is, it increases with decreasing atomic size, with decreasing principal quantum number, depends on the type of EO hybridization and on some other factors. The most important ligands can be arranged in a series of increasing “strength” (a kind of “activity series” of ligands), this series is called spectrochemical series of ligands:

I; Br ; : SCN, Cl, F, OH, H2O; : NCS, NH 3; SO 3 S : 2 ; : CN, CO

For complexes 3 and 3, the formation schemes are as follows:

For complexes with CN = 4, two structures are possible: tetrahedron (in the case sp 3-hybridization), for example, 2, and a flat square (in the case dsp 2-hybridization), for example, 2.

17.5. Chemical properties of complex compounds

Complex compounds are primarily characterized by the same properties as ordinary compounds of the same classes (salts, acids, bases).

If the complex compound is an acid, then it is a strong acid; if it is a base, then it is a strong base. These properties of complex compounds are determined only by the presence of H 3 O or OH ions. In addition, complex acids, bases and salts enter into ordinary exchange reactions, for example:

SO 4 + BaCl 2 = BaSO 4 + Cl 2
FeCl 3 + K 4 = Fe 4 3 + 3KCl

The last of these reactions is used as qualitative reaction to Fe 3 ions. The resulting ultramarine-colored insoluble substance is called “Prussian blue” [systematic name: iron(III)-potassium hexacyanoferrate(II).

In addition, the complex particle itself can enter into a reaction, and the more active it is, the less stable it is. Typically these are ligand substitution reactions occurring in solution, for example:

2 + 4NH 3 = 2 + 4H 2 O,

as well as acid-base reactions such as

2 + 2H 3 O = + 2H 2 O
2 + 2OH = + 2H 2 O

The product formed in these reactions, after isolation and drying, turns into zinc hydroxide:

Zn(OH) 2 + 2H 2 O

The last reaction is the simplest example of the decomposition of a complex compound. In this case, it occurs at room temperature. Other complex compounds decompose when heated, for example:

SO4. H 2 O = CuSO 4 + 4NH 3 + H 2 O (above 300 o C)
4K 3 = 12KNO 2 + 4CoO + 4NO + 8NO 2 (above 200 o C)
K 2 = K 2 ZnO 2 + 2H 2 O (above 100 o C)

To assess the possibility of a ligand substitution reaction, a spectrochemical series can be used, guided by the fact that stronger ligands displace less strong ones from the inner sphere.

17.6. Isomerism of complex compounds

Isomerism of complex compounds is associated
1) with possible different arrangements of ligands and outer-sphere particles,
2) with a different structure of the complex particle itself.

The first group includes hydrate(in general solvate) And ionization isomerism, to the second - spatial And optical.

Hydrate isomerism is associated with the possibility of different distribution of water molecules in the outer and inner spheres of a complex compound, for example: (red-brown color) and Br 2 (blue color).

Ionization isomerism is associated with the possibility of different distributions of ions in the outer and inner spheres, for example: SO 4 (purple) and Br (red). The first of these compounds forms a precipitate by reacting with a solution of barium chloride, and the second with a solution of silver nitrate.

Spatial (geometric) isomerism, otherwise called cis-trans isomerism, is characteristic of square and octahedral complexes (impossible for tetrahedral ones). Example: cis-trans isomerism of a square complex

Optical (mirror) isomerism is essentially no different from optical isomerism in organic chemistry and is characteristic of tetrahedral and octahedral complexes (impossible for square ones).