In chemistry, dissociative substitution describes a reaction pathway by which compounds interchange ligands.  The term is typically applied to coordination and organometallic complexes, but resembles the S1 mechanism in organic chemistry.  This pathway can be well described by the cis effect, or the labilization of CO ligands in the cis position. The opposite pathway is associative substitution, being analogous to S2 pathway. Pathways that are intermediate between the pure dissociative and pure associative pathways are called interchange mechanisms.
Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry. 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.