Associative substitution

[1][2] Associative pathways are characterized by binding of the attacking nucleophile to give a discrete, detectable intermediate followed by loss of another ligand.

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.

The electrostatically held nucleophile can exchange positions with a ligand in the first coordination sphere, resulting in net substitution.

The rate for the hydrolysis of cobalt(III) ammine halide complexes are deceptive, appearing to be associative but proceeding by an alternative pathway.

Studies show, however, that the hydroxide deprotonates one NH3 ligand to give the conjugate base of the starting complex, i.e., [Co(NH3)4(NH2)Cl]+.

[6][7] The key feature of the mechanism is an initial rate-determining pre-equilibrium to form an encounter complex ML6-Y from reactant ML6 and incoming ligand Y.

The Eigen-Fuoss equation shows that higher values of KE (and thus a faster pre-equilibrium) are obtained for large, oppositely-charged ions in solution.