Clasificación de los almacenes: Según su relación con el flujo de

The most fundamental substitution reaction in aqueous solution, water exchange, reaction (22), has been studied for a variety of metal ions (Figure 6.3).

Exchange of water in the coordination sphere of a metal with bulk solvent water occurs very rapidly for most metal ions, and therefore the rates of these reactions were studied primarily by relaxation techniques. In these methods, a system at equilibrium is disturbed, for example, by a very sudden increase in temperature.

Under the new condition—higher temperature—the system will no longer be at equilibrium. The rate of equilibration can then be measured. If one can change the temperature of a solution in 10^–8 s, then one can measure the rates of reactions that take longer than 10^–8 s.

In Section 6.4, it was noted that the rate of water exchange decreases markedly as the charge on the metal ion increases and its size decreases. An increased charge-to-radius ratio is expected to strengthen metal–ligand bonds

which the anion has replaced a water molecule, reaction (27).

(27) For a variety of metal ions it has been found that the rate of reaction (27) is similar to its rate of water exchange; the nature of the entering ligand has little influence on the rate. Therefore, bond formation to the entering ligand apparently contributes little to the energetics of the rate-determining step. This and the fact that the rate is similar to water exchange implies that a similar Idmechanism is involved for both water exchange and water substitution by an entering ligand.

Probably, the most widely studied coordination compounds are the ammine complexes of cobalt(III). Their stability, ease of preparation, and slow reactions makes them particularly amenable to kinetic study. Since work on these complexes has been done almost exclusively in water, the reactions of the complexes with the solvent water had to be considered first. In general, ammonia or amines coordinated to cobalt(III) are observed to be replaced so slowly by water that only the replacement of ligands other than amines is usually considered.

The rates of reactions of the type of reaction (28) have been studied and [CoX(NH3)5]²⁺ + H2Oĺ [CoOH2(NH3)5]³⁺ +X^– (28) found to be first order in the cobalt complex. (X can be any of a variety of anions.) Since in aqueous solution the concentration of H2O is always about 55.5 M, the effect of changes in water concentration on the reaction rate cannot be determined. Rate laws (29) and (30) are experimentally indis-

rate = k[CoX(NH3)52+] (29)

rate = kƍ[CoX(NH3)5

2+][H2O] (30)

tinguishable in aqueous solution, since k may simply be kƍ[H2O] = kƍ[55.5].

Therefore, the rate law does not tell us whether H2O is involved in the rate-

determining step of the reaction. The decision as to whether these reactions proceed by displacement of X by H2O or by dissociation followed by addition of H2O must be made from other experimental data.

Two of the many types of experiments which have provided good mechanistic evidence are presented here. The rate of hydrolysis (replacement of one chloride by water) of trans-[CoCl2(NH3)4]⁺ is approximately 10³times faster than that of [CoCl(NH3)5]²⁺. Since a decrease in rate is observed as the charge on the complex increases, a dissociative process seems to be operative.

Another piece of evidence results from the study of the hydrolyses of a series of complexes related to trans-[CoCl2(en)2]⁺. In these complexes, the ethylenediamine is replaced by similar diamines in which H atoms on C were replaced by CH3 groups. The complexes containing substituted diamines react more rapidly than the ethylenediamine complex. The replacement of H by CH3

increases the bulk of the ligands. Models of these compounds show that this should make it more difficult for an attacking ligand to approach the metal atom.

This steric crowding should retard an associative reaction. By crowding the vicinity of the metal atom with bulky ligands, one enhances a dissociative process, since removal of one ligand reduces congestion around the metal. The increase in rate observed when the more bulky ligands were used is good evidence for a dissociative mechanism.

As a result of a large number of studies on acido amine complexes of cobalt(III), it appears that replacement of the acido group by H2O occurs by a process that is primarily dissociative in character. The ligand–cobalt bond must be stretched to some critical distance before a H2O molecule begins to enter the coordination sphere.

Replacement of an acido group (X^–) in a cobalt(III) ammine with a group other than H2O, reaction (31), has been observed to take place by initial

[CoX(NH3)5]²⁺ +Y^–ĺ [CoY(NH3)5]²⁺ + X^– (31) substitution by solvent H2O with subsequent replacement of water by the new group Y, reaction (32). Therefore, in a number of cobalt(III) reactions

(32) the rates of reaction (31) are the same as the rate of hydrolysis, reaction (28).

Hydroxide ion is different from other reagents with respect to its reactivity toward cobalt(III) ammine complexes. It reacts very rapidly (as much as 10⁶times faster than H2O) with cobalt(III) ammine complexes in a base hydrolysis reaction (33). In this reaction, a first-order dependence on the substituting

[CoCl(NH3)5]²⁺ + OH^–ĺ [CoOH(NH3)5]²⁺ +Cl^– (33) ligand OH^– is observed, equation (34). The second-order kinetics and

Brønsted acid to give [Co(NH3)4NH2Cl] , which is known as an amido (: H2

containing) compound and which is the conjugate base of [CoCl(NH3)5]²⁺. The reaction then proceeds by a dissociative process, reaction (36), to give a five-coordinated intermediate which reacts with solvent to give the observed product in reaction (37). This mechanism is consistent with second-order rate behavior, and yet it involves a dissociative mechanism. Since the reaction involves the conjugate base of the initial complex in an Idrate-determining step, the term IdCB is given to the mechanism.

The task of determining which mechanism best explains the experimental observations is difficult. However, there is convincing evidence to support the IdCB hypothesis. When a reaction such as (33) is carried out in the presence of a relatively large concentration of a ligand Y^–, some [CoY(NH3)5]²⁺ is formed. The amount formed relative to [CoOH(NH3)5]²⁺ is independent of the OH^– concentration (in basic solution) and increases as the concentration of Y^– is raised. Direct displacement of a ligand by OH^– does not explain why Y^– should be incorporated, whereas a reactive five-coordinate intermediate as in reaction (36) would be expected to react with Y as well as H2O. Such competition experiments show that an Ia mechanism is not correct for these reactions. The IdCB mechanism is not proved to be true, but continues as a plausible mechanism. (It is not possible to prove that a mechanism is correct, but it is possible to prove one wrong.)

Another piece of evidence to support the IdCB mechanism is that if there is no N–H hydrogen present in a cobalt(m) complex, the complex reacts slowly with OH^–. This suggests that acid–base properties of the complex are more important to the rate of reaction than are the nucleophilic properties of OH^–. This base hydrolysis reaction of cobalt(III) ammine complexes illustrates the fact that kinetic data often can be interpreted in more than one way and that rather subtle experiments must be performed to eliminate one or more possible mechanisms.

Substitution reactions of a wide variety of octahedral compounds have now been studied. Where mechanistic interpretation of the data has been made, a dissociative-type process has most frequently been postulated. This result

should not be surprising, since six ligands around a central atom leave little room for adding another group. Nonetheless, evidence that entering groups do influence reaction rates is frequently found, and Idmechanisms in which bond breaking is more important than bond making are common. There are also examples where bond making contributes significantly to the energetics, and therefore associative mechanisms cannot be discarded as paths for octahedral substitution.