Described are multiple approaches using density functional theory to probe the acid catalyzed aquation of the hexaammineruthenium(II) cation (Ru(NH3)62+ + H3O+ → Ru(NH3)5(H2O)2+ + NH4+) reported initially by Taube and co-workers. These computations support the proposal that the initial step is protonation of the Ru(II) center and/or the metal-NH3 bond, thereby activating the latter toward dissociation. DFT analysis was also carried out for the hypothetical acid-mediated aquation of the isoelectronic hexaamminerhodium(III) complex, Rh(NH3)63+. The computations suggest a key mechanistic difference for the latter pathway, namely that protonation of the NH3 occurs late in a reaction coordinate involving dissociation of the Rh-NH3 with no direct interaction of H+ with the metal center. Furthermore, while the calculated activation energy is considerably higher in the latter case, the calculations suggest that protonation could play an important role in such ligand substitution reactions.
Laverman et al. (2026) studied this question.