Kinetics and Mechanism of Oxidation of S 2 O 3 2− by a Co‐Bound μ ‐Amido‐ μ ‐Superoxo Complex

In acetate buffer media (pH 4.5–5.4) thiosulfate ion (S 2 O 3 2− ) reduces the bridged superoxo complex, [(NH 3 ) 4 Co III (μ‐NH 2 ,μ‐O 2 )Co III (NH 3 ) 4 ] 4+ ( 1 ) to its corresponding μ‐peroxo product, [(NH 3 ) 4 Co III (μ‐NH 2 ,μ‐O 2 )Co III (NH 3 ) 4 ] 3+ ( 2 ) and along a parallel reaction path, simultaneously S 2 O 3 2− reacts with 1 to produce the substituted μ‐thiosulfato‐μ‐superoxo complex, [(NH 3 ) 4 Co III (μ‐S 2 O 3 ,μ‐O 2 )Co III (NH 3 ) 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 S 2 O 3 2− group. In reaction media, 3 readily dissolves to further react with S 2 O 3 2− to produce μ‐thiosulfato‐μ‐peroxo product, [(NH 3 ) 4 Co III (μ‐S 2 O 3 ,μ‐O 2 )Co III (NH 3 ) 4 ] 2+ ( 4 ). The observed rate ( k 0 ) increases with an increase in [T Thio ] ([T Thio ] is the analytical concentration of S 2 O 3 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 k 1 and k 2 and the change of absorbance is equal to { a 1 exp(– k 1 t ) + a 2 exp(– k 2 t )}, where k 1 > k 2 .