Electron Transfer Reactions of an Osmium (II) Polypyridyl Complex with Oxidizing Agents

Redox rates between polypyridyl osmium (II) complex, OsCl(py)3(bipy)+, with tris(1,10-phenanthroline) cobalt (III), Co(phen)33+, and Ce(IV) were examined in aqueous solution. Both reaction systems exhibited second order kinetics. The osmium (II) - cobalt (III) redox reaction was found to have a rate constant of 2.3 x 105 M-1 sec-1 at 22°C and µ = 0.003M from which the isotopic electronic exchange rate constant for the OsCl(py)3(bipy)+/2+ system was estimated to be 1.5 x 109 M-1sec-1. Ionic strength effects on the Ce(IV) - Os(II) redox reaction were studied as well as the effects of Ce(IV) and NaN03 concentrations on the solution pH. The redox rate constant was 52 M-1sec-1 at 22°C and µ = 0.01M. The reaction rate decreased with increasing ionic strength and a Bronsted plot was consistent with Ce(IV) present as Ce(NO3)5(OH)23-. Activation parameters were determined and a mechanism of electronic transfer was proposed for each system. The Os(II) - Co(III) redox reaction is believed to occur via an outer-sphere mechanism while an inner-sphere mechanism was proposed for the Os(II) - Ce(IV) system. Since relatively high acid concentrations were used in the Ce(IV) - Os(II) study, the acid hydrolysis rate of the Os(II) complex was measured and found to be sufficiently slow so as not to interfere with the redox rate. This reaction exhibited third-order kinetics