Mechanism and Stereochemistry of the Water-Exchange Reaction on Aqua Pentaammine and Aqua Pentakis-Methylamine Rhodium(III) Ions

The water-exchange reaction on Rh(NH3)5OH2 3+ and Rh(NH2CH3)5OH2 3+ has been investigated by ab initio calculations on all stationary points of the potential energy surface (reactants/products, transition states, and intermediates, if there are any). The water-exchange on Rh(NH3)5OH2 3+ proceeds via the I (interchange) mechanism, whereas that on Rh(NH2CH3)5OH2 3+ follows a dissociative (Id or D) pathway because of the bulky NH2CH3 ligands. Both reactions proceed with retention of the configuration. For the ammonia complex, the computed activation energy agrees with experiment, whereas for the methylamine one, equal activation energies were found for the Id and the D pathways, and they are lower than ΔH 298 ⧧ by 22 kJ/mol. The distinction of these two mechanisms is not possible on the basis of the present calculations, and the limitations of the model are discussed. The relation between the electronic structure of the transition state for the interchange mechanism and the intrinsic component of the activation volume is analyzed via the comparison of the water-exchange reactions on the aqua pentaammine complexes of RuIII and RhIII. For these two very similar exchange reactions, which give rise to equal experimental activation volumes (ΔV 298 ⧧), the intrinsic and electrostrictive components are most likely quite different.