Question

define dissociative, ID, IA and associative substitution mechanisms

Answer 1

Ligand substitution is characterized by a continuum of mechanisms bound by associative (A) and dissociative (D) extremes.

At the associative extreme, the incoming ligand first forms a bond to the metal, then the departing ligand takes its lone pair and leaves. It is common for 16-electron complexes (like d8 complexes of Ni, Pd, and Pt),

At the dissociative extreme, the order of events is opposite—the departing ligand leaves, then the incoming ligand comes in. It is the norm for 18-electron complexes.

In some cases, the evidence is available for simultaneous dissociation and association, and this mechanism has been given the name interchange (IA or ID).