Ligand Exchange

Ligand exchange reactions are of the kind 

and are efficiently nucleophilic substitutions. The probable techniques are categorized as associative (A) or dissociative (D) in accordance with whether the new bond is created before or after the old one is broken, or interchange (I), the intermediate case, that can be subdivided into I_a or I_d in accordance with the degree of associative or dissociative character. Kinetic studies of ligand swap can occasionally distinguish among the techniques even though these results can be misleading. Purpose of the entropy or volume of activation (that is the volume or entropy alter in the transition state) can frequently provide guidance.

For several metal ions ligand exchange is a very fast reaction, along with rate constants close to the limit of diffusion control (approximately 10¹⁰ M^-1 s^-1 in water). There is a association with the charge and size and outside the transition series Be²⁺ and Al³⁺, that contain large charge/size ratio, have considerably slower exchange. With transition metals the affect of ligand field effects is noticeable. Complexes of Cr³⁺ (d³) and Co³⁺ (d⁶) and of several d6 and d8 ions within the 4d and 5d series are kinetically inert and go through ligand substitution several orders of magnitude more slowly than comparable nontransition ions. These ions contain a ligand field stabilization energy (LFSE) which contributes a obstacle to the geometrical change needed in the transition state. A large LFSE value also provides shorter bond lengths that improve other contributions (electrostatic, etc.) to the metal-ligand bond strength.

The presence of kinetically inert complexes is helpful in mechanistic studies, and significant for the separation of dissimilar isomers.