Associative A Mechanism

In the associative mechanism, which is given by Equations (17.12) and (17.13), the influence of the entering ligand on the rate constant is immediately evident in the equation for the rate law. Assuming that the RDS in the mechanism involves an equilibrium with association of Y to form a seven-coordinate intermediate and that the ensuing dissociation of X to form product is rapid, the rate law for an associative mechanism can be determined as follows  

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At the other extreme is the associatively (a) activated associative (A) mechanism, in which the rate-determining step for substitution by 1/ proceeds through a reactive intermediate of increased coordination number, [M(H20) L](m x,+, which has normal vibrational modes and survives several molecular collisions before losing H20 to form [M(H20) 1L](m t,+, as shown in Eq. (8). Equation (9) indicates the linear variation with excess [I/-] anticipated for obs, which is similar in form to that of Eq. (5) when if0[I/ ] 1 and kohs + k. ... 

Associative (A) mechanisms are extremely rare and it is uncertain whether an authentic example exists. Dissociative (D) mechanisms are more common although difficult to establish. Some examples were cited in Secs. 4.2.5 and 4.2.6. Thus interchange ( ) mechanisms dominate the scene. This leads to the following generalizations ... 

A truely associative (A) mechanism will give a simple second-order rate law as long as the concentration of the intermediate of higher coordination number remains small. 

For several metal ions like Ni2+, the exchange rate constant s entropy of activation is positive, and ligand properties do not appreciably affect the rate of substitution then, the solvent exchange rate is a characteristic of the metal ion, and the mechanism is dissociative (D). In contrast, associative (A) mechanisms have negative entropies of activation the substitution rate constants vary with ligand, and a characteristic substitution rate constant is not a meaningful concept. 

In summary, the results obtained by Rotzinger on first-row transition metals provided the following picture [41]. Hexacoordinated Sc " ", Ti and react via an associative A mechanism with relatively long-lived intermediates. The dissociative pathway is only possible for water exchange on Ni, Cu + and Zn. For the elements in the middle of the 3d series both associative (la/A) and dissociative (D) pathways can occur. 

Chromium(ni) continues to be the second most widely studied metal ion after cobalt(m). The question of dissociative (/a) or associative (/a) mechanisms for substitutions at chromium(m) has been clarified somewhat by comparing data for the formation and dissociation of the [Cr(H20)5X] + and [Cr(NH8)5X] + ions (X = unidentate leaving anion). Recent studies indicate that an associative mechanism is important for aqua-chromium(m) complexes, but for the [Cr(NHa)6X] + ions a dissociative-interchange mechanism is favoured. A summary of kinetics and thermodynamic data for the formation and aquation of [Cr(NH3)5X] + ions is given in Table 12. 

Non-aqueous Solvents.—Mention has already been made of the evidence for an associative (/a) mechanism for the exchange of DMSO with the [Cr(DMSO) ] + ion. Both the entropy and volume of activation are large and negative. A recent study of the exchange process by n.m.r. in DMSO-MeNOj mixed solvents (nitro-methane is an inert, non-co-ordinating diluent which is known to have very little rate effect) shows that the exchange rate is approximately constant above 0.2 mole fraction of DMSO, but drops off sharply below this concentration. On the other hand the fraction of DMSO molecules in the solvation shell of the [Cr(DMSO) J + ion decreases immediately with the decrease in the DMSO mole fraction. This difference in concentration effects is postulated to arise from a unique outer-sphere solvation site which preferentially binds DMSO molecules and preferentially participates in the exchange reaction. 

How might this substitution actually take place on the molecular level Or, to restate the question, what is the reaction mechanism—that is, the sequence of molecular-level steps involved in the reaction At first blush, there are at least two major possibilities the dissociative (D) mechanism and the associative A) mechanism. 

On the other hand, if the overall process (1.9.1) follows an associative (A) mechanism, identical with the Sj.j2(lim.) mechanism in Ingold s symbolism, then the reaction scheme is ... 

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