The oxidation of hydrogen peroxide by tris(polypyridine) complexes of osmium(III), iron(III), ruthenium(III), and nickel(III) in aqueous media

The stoichiometry and kinetics of the oxidation of hydrogen peroxide by tris(2,2′-bipyridine) and tris(4,4′-dimethyl-2,2′-bipyridine) complexes of osmium(III), iron(III), ruthenium(III), and nickel (III) were studied in acidic and neutral aqueous media at 25 °C and I = 0.50 M (LiCF 3 SO 3 ). The reaction 2M(bpy) 3 3+  + H 2 O 2  → 2M(bpy) 3 2+  + O 2  + 2H + is observed with quantitative yields of dioxygen gas. The observed rate constants displayed an inverse acid dependence over the pH range 6.0–8.5; k obsd  = k 1  + k 2 K 1 /[H + ], attributed to the oxidations of H 2 O 2 (k 1 ) and HO 2 − (k 2 ). An application of the Marcus theory relationship to the cross-reaction data gave a self-exchange rate constant of 10 −2 –10 −1  M −1 s −1 for the HO 2 − /HO 2 couple. The electron exchange rate constant is evaluated in terms of the inner-sphere and solvent reorganizational barriers and is compared to values reported for other small molecule couples. Rate and activation parameters for the reduction of the nickel(III) complexes by the hydroxide ion have been determined and are compared with the corresponding values for other metal tris(poly pyridine) complexes.