The term is typically applied to coordination and organometallic complexes, but resembles the SN1 mechanism in organic chemistry.
The entropy of activation is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate-determining step.
Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution.
If the reaction rate is insensitive to the nature of the attacking nucleophile, the process is called dissociative interchange, abbreviated Id.
An illustrative process comes from the "anation" (reaction with an anion) of cobalt(III) complexes:[3] The exchange between bulk and coordinated water is of fundamental interest as a measure of the intrinsic kinetic lability of metal ions.
For octahedral mono- and dicationic aquo complexes, these exchange processes occur via an interchange pathway that has more or less dissociative character.
The rate for the hydrolysis of cobalt(III) ammine (NH3-containing) halide complexes are deceptive, appearing to be associative but proceeding by a pathway that is dissociative in character.
Studies show, however, that in the hydroxide deprotonates one NH3 ligand to give the conjugate base of the starting complex, i.e., [Co(NH3)4(NH2)Cl]+.