Kinetics and Mechanism of Oxidation of S2O32- by a Co-Bound μ-Amido-μ-Superoxo Complex.

In acetate buffer media (pH 4.5-5.4) thiosulfate ion (S₂O₃^2-) reduces the bridged superoxo complex, [(NH₃)₄Co^III(μ-NH₂,μ-O₂)Co^III(NH₃)₄]⁴⁺ (1) to its corresponding μ-peroxo product, [(NH₃)₄Co^III(μ-NH₂,μ-O₂)Co^III(NH₃)₄]³⁺ (2) and along a parallel reaction path, simultaneously S₂O₃^2- reacts with 1 to produce the substituted μ-thiosulfato-μ-superoxo complex, [(NH₃)₄Co^III(μ-S2O3,μ-O₂)Co^III(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 S₂O₃^2- group. In reaction media, 3 readily dissolves to further react with S₂O₃^2- to produce μ-thiosulfato-μ-peroxo product, [(NH₃)₄Co^III(μ-S2O3,μ-O₂)Co^III(NH₃)₄]²⁺ (4). The observed rate (k₀) increases with an increase in [T_Thio] ([T_Thio] is the analytical concentration of S₂O₃^2-) and temperature (T), but it decreases with an increase in [H⁺] and the ionic strength (I). Analysis of the log A_t 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. [ABSTRACT FROM AUTHOR]