Nuclear magnetic resonance study of the proton exchange reaction of hexaammineruthenium(III)

Water nmr line widths, shifts, and longitudinal relaxation times due to the presence of dissolved Ru(NH₃)₆Cl₃ were measured. The marked pH and temperature dependence of these quantities was explained by a base-catalyzed chemical exchange of the ammine protons of the complex ions. The measured parameters for the apparent second-order rate constant were k = 2.0 × 10⁹ l. (g-atom)^-1 sec^-1 at 25°, ΔH* = 20.0 kcal mol^-1, and ΔS* = 51 eu, based on the exchange rate of a single NH proton. An assumption of a single-step exchange mechanism leads to a bimolecular rate constant of 18k = 3.6 × 10¹⁰ M^-1 see^-1 for the reaction between Ru(NH₃)₆³⁺ and OH^-. This value is close to the diffusion controlled limit and is incompatible with the high observed activation energy. A multistep mechanism was postulated which involves formation of ion pairs and deprotonated species as the first two steps followed by proton exchange, between the deprotonated species and the bulk water, which might be assisted by a Grot-thuss type mechanism. Small values of chemical shifts and longitudinal relaxation rates of water protons were observed in the absence of proton exchange with the complex, and were tentatively assigned to water molecules in the second coordination sphere.