Kinetics and Mechanism of Oxidation of S2O32− by a Co‐Bound μ‐Amido‐μ‐Superoxo Complex

Abstract

ABSTRACTIn acetate buffer media (pH 4.5–5.4) thiosulfate ion (S2O32−) reduces the bridged superoxo complex, [(NH3)4CoIII(μ‐NH2,μ‐O2)CoIII(NH3)4]4+ (1) to its corresponding μ‐peroxo product, [(NH3)4CoIII(μ‐NH2,μ‐O2)CoIII(NH3)4]3+ (2) and along a parallel reaction path, simultaneously S2O32− reacts with 1 to produce the substituted μ‐thiosulfato‐μ‐superoxo complex, [(NH3)4CoIII(μ‐S2O3,μ‐O2)CoIII(NH3)4]3+ (3). The formation of μ‐thiosulfato‐μ‐superoxo complex (3) appears as a precipitate which on being subjected to FTIR shows absorption peaks that support the presence of Co(III)‐bound S‐coordinated S2O32− group. In reaction media, 3 readily dissolves to further react with S2O32− to produce μ‐thiosulfato‐μ‐peroxo product, [(NH3)4CoIII(μ‐S2O3,μ‐O2)CoIII(NH3)4]2+ (4). The observed rate (k0) increases with an increase in [TThio] ([TThio] is the analytical concentration of S2O32−) and temperature (T), but it decreases with an increase in [H+] and the ionic strength (I). Analysis of the log At versus time data (A is the absorbance of 1 at time t) reveals that overall the reaction follows a biphasic consecutive reaction path with rate constants k1 and k2 and the change of absorbance is equal to {a1 exp(–k1t) + a2 exp(–k2t)}, where k1 > k2.