Associative Substitution

Associative substitution describes a pathway by which compounds interchange ligands. The terminology is typically applied to coordination and organometallic complexes, but resembles the Sn2 mechanism in organic chemistry. The opposite pathway is dissociative substitution, being analogous to Sn1 pathway. Intermediate pathways exist between the pure associative and pure dissociative pathways, these are called interchange mechanisms.

Associative pathways are characterized by binding of the attacking (nucleophile) to give a discrete, detectable intermediate followed by loss of another ligand. Complexes that undergo associative substitution are either coordinatively unsaturated or contain a ligand that can change its bonding to the metal, e.g. change in hapticity or bending of NO. In homogeneous catalysis, the associative pathway is desirable because the binding event, and hence the selectivity of the reaction, depends not only on the nature of the metal catalyst but also on the substrate.

Examples of associative mechanisms are commonly found in the chemistry of 16e square planar metal complexes, e.g. Vaska's complex and tetrachloroplatinate. These compounds (MX₄) bind the incoming (substituting) ligand Y to form pentacoordinate intermediates MX₄Y that in a subsequent step dissociates one of their ligands. Dissociation of Y results in no reaction, but dissociation of X results in net substitution, giving the 16e complex MX₃Y. The first step is typically rate determining. Thus, the entropy of activation is negative, which indicates an increase in order in the system. These reactions follow second order kinetics: the rate of the appearance of product depends on the concentration of MX₄ and Y. The rate law is governed by the Eigen–Wilkins Mechanism.