Mechanisms of ligand substitution reactions general considerations

3 Mechanisms of ligand substitution reactions general considerations 

An additional complication may be that the reaction coordinate may well not be the smooth, continuous, motion implied by this diagram. So, for example, the motions that lead from reactants to reaction intermediate may be rather different from those involved in the passage from reaction intermediate to products. 

A little consideration soon shows that Fig. 14.6 is likely to be somewhat idealized. It implies that the path from products to reactants is smooth and uneventful. There are no sticky spots. But consider the reaction between a cationic complex, [Ni(H20)6] for example, and an anion, C say, to give the complex ion [Ni(H20)5Cl]. Simple electrostatic considerations suggest that the ion pair [Ni(H20)6] Cl may well have some stability, particularly in solvents with a relatively low dielectric constant. The reactants are sticky the ion pair is likely to exist until atomic positions and momenta are either such as to allow the reaction to proceed or the ion pair to dissociate. Such an intermediate, formed between the reactants but in advance of reaction between them, is called a precursor complex. Similarly, when the reaction is one which involves loss of an anionic species by the complex, a so-called successor complex may be an intermediate on the way to the final product. This pattern is shown in Fig. 14.7 which also includes the possibility of a reaction intermediate of some stability. Fig. 14.7 shows a situation which is complicated and would therefore have a complicated rate law. Most systems studied either are, or are assumed to be, rather simpler. 

Despite its complexity. Fig. 14.7 begs one important question. Consider either the reaction intermediate in that figure or, when no such intermediate is formed, the corresponding transition state (shown dotted in Fig. 14.7). Compared to the coordination number of the metal in the reactant complex, at this point in the potential energy surface the coordination number of the metal could have increased by one, decreased by one or stayed the same. The rate-determining step in the reaction mechanism involves association, dissociation or an interchange, respectively, and these labels. A, D and I, are used to describe the reaction type. The job of classifying a particular 

Type D mechanisms imply an increase in the effective number of particles in the system, an increase in the number of possible arrangements, and so AS is positive. Conversely, negative AS values imply A mechanisms. Type I mechanisms are associated with very small magnitudes of AS. However, the view just adopted is somewhat simplistic in that there are many contributions to the AS —translational, rotational, vibrational—of the reactants, together with a contribution from solvent ordering, and it is their sum which is determined. For this reason, and because a long graphical extrapolation is needed to obtain them, AS values are perhaps less useful than AV. However, they are easier to obtain. 

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