Kinetics and mechanism of the reduction of diaquotetrakis (2,20-bipyridine)-l-oxodiruthenium(III) ion by hypophosphorous acid in acidic medium

The kinetics and mechanism of the reduction of diaquotetrakis(2,20-bipyridine)-l-oxodiruthenium(III), [(H2O)2(bipy)4Ru2O]4?, by H3PO2 has been studied in aqueous acid at ionic strength = 0.5 mol dm-3 (NaClO4), [H?] = 5.0 9 10-2 mol dm-3 and temperature = 31 ± 1 �C. Measurement of the stoichiometry showed that 1 mole of [(H2O)2(bipy)4Ru2O]4? was reduced by 1 mole of H3PO2. The reaction was found to be first order with respect to both [(H2O)2(bipy)4Ru2O4?] and [H3PO2], hence second order overall. Variations in the ionic strength and dielectric constant of the reaction medium had no effect on the rate. Also, addition of various ions to the reaction medium did not significantly alter the rate. Free radicals were identified during the course of the reaction by a polymerisation test. Spectroscopic information and Michaelis–Menten plots suggested the absence of an intermediate complex prior to electron transfer. [(H2O)2 (bipy)2Ru]2?, the reduction product of [(H2O)2(bipy)4 Ru2O]4?, plus H3PO3, the oxidation product of H3PO2, were identified in the product solutions. It is suggested that the reaction proceeds through the outer sphere pathway. A mechanism for the reaction is proposed.