Octahedral Complexes 

Several M²⁺ ions of the 3d series go through ligand exchange at a rate comparable with that for nontransition metal ions of identical size. V²⁺ (d³) and Ni²⁺ (d⁸) are rather slower, these being the electron configurations which provide maximum octahedral LFSE for high-spin ions. Volumes and Entropies of activation imply a change from predominantly I_a techniques early in the series (example V²⁺) to I_d in the direction of the end (e.g. Ni²⁺). Both decreasing size and increasing d orbital occupancy might contribute to this tendency. Incoming ligands in the Ia mechanism might be approach an octahedral complex along directions in which the t₂g orbitals generally point. Filling these orbitals will tend to reduce the approach of ligands and favor the dissociative pathway.

For the kinetically inert low-spin Co^III complexes the technique of exchange is specifically dissociative even though kinetic studies can provide results which are super-ficially misleading. For instance, the base hydrolysis reaction

Have a rate proportional to the concentrations of both complex and OH-. This is not suggestive of an associative technique, but of a conjugate base technique in which the first reversible step is deprotonation of the complex:

Deprotonation trans to the leaving group is particularly efficient at promoting the dissociation step. The conjugate base technique cannot operate if a tertiary amine along with no ionizable proton is located trans to the leaving group; as supposed the rate of substitution is then slower and does not rely on [OH^-].