Dissociative substitution

Dissociative substitution describes a pathway by which compounds interchange ligands. The term is typically applied to coordination and organometallic complexes, but resembles the Sn1 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 Sn2 pathway. Intermediate pathways exist between the pure dissociative and pure associative pathways, these are called interchange mechanisms. Dissociative substitution describes a pathway by which compounds interchange ligands. The term is typically applied to coordination and organometallic complexes, but resembles the Sn1 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 Sn2 pathway. Intermediate pathways exist between the pure dissociative and pure associative pathways, these 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. Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting nucleophile has no influence on this rate, and an intermediate of reduced coordination number can be detected. The reaction can be described with k1, k−1 and k2, which are the rate constants of their corresponding intermediate reaction steps: