Ligand substitution

Replacement of one coordinated ligand by another. Formal metal oxidation state need not change, even though local electron count can change while association or dissociation occurs.

A donor tethered to a complex captures a vacant coordination site created by coordination ligand loss. Its effective proximity can compete favourably with weakly coordinating bulk solvent, provided its geometry allows it to reach the metal.

Photoexcitation can dissociate a carbonyl ligand, creating a coordinatively unsaturated metal fragment. Capture by solvent or a tethered donor competes with CO recombination; net substitution therefore depends on both the photochemical step and subsequent trapping.

Substitution promoted by prior deprotonation of a coordinated ligand. In cobalt(III) ammine complexes, an amido donor can assist departure of another ligand; solvent capture and reprotonation complete the substitution.