dissociative substitution mechanism (D)

Back to the crowded bench, but now the swap happens the other way around: someone gets up and leaves first, the bench is briefly emptier, and only then does the newcomer drop into the open seat. The dissociative mechanism is leave-first: the old ligand departs before the new one arrives, so for a moment the metal is less crowded than usual.

In a dissociative (D) substitution, the leaving ligand breaks away first to give a real intermediate with a reduced coordination number — an octahedral six-coordinate complex briefly becomes a five-coordinate species — and that hungry, undersaturated intermediate then grabs the incoming ligand. The kinetic fingerprint is the mirror image of the associative case: the slow, rate-determining step is the departure of the leaving group, which involves only the complex itself, so the rate does not depend on the concentration of the incoming ligand. You see first-order kinetics, rate equals k times the complex. And because the transition state is looser and more spread out than the starting complex, the volume of activation is positive — the system expands as the bond stretches and breaks, so high pressure slows the reaction down.

The dissociative pathway dominates when the metal is already crowded and reluctant to take on a seventh ligand, which describes most octahedral complexes. Bulky ligands, a high coordination number, and a metal centre that gains little from extra bonding all push toward leave-first behaviour. This is the everyday mechanism behind water exchange and ligand substitution at six-coordinate metal ions across the periodic table. As always, be honest about the continuum: a clean D mechanism with a truly isolable reduced-coordination intermediate is one idealized end, and many octahedral substitutions actually proceed by dissociative interchange (Id), where bond-breaking leads but no genuine intermediate ever fully forms.