Metal-ion oxidation in solution. Part IV. The oxidation of thiomalic acid by cobalt(III) ions

Abstract

The rate of oxidation of thiomalic acid by cobaltic ions has been studied over the temperature range 0–17° using a stopped-flow technique. Measurements of the rate of formation of the transient intermediate complex formed in the course of the reaction have been made, and its ultraviolet spectrum has been obtained. The decrease in the absorbance due to the cobaltic ion at 600–610 mµ has been used to measure the electron-transfer reaction. The rate constants k₁ and k₂ for the reactions: Co³⁺+ RSH [graphic omitted] [Co(RSH)]³⁺ and Co(OH)²⁺+ RSH [graphic omitted] [Co(OH)(RSH)²⁺(RSH = thiomalic acid) are 8·2 ± 2·1M^–1 sec.^–1 and 320 ± 50M^–1 sec.^–1, respectively, at 7° and µ0·25M. The observed rate constant for the oxidation reaction may be expressed as k_obs(electron-transfer)= 0·05 + 0·27/[H⁺] at 12·7° and µ1·0M. Temperature studies yield activation energies of 37·1 ± 1·5 and 15·2 ± 1·0 kcal./mole for the oxidations involving Co(RSH)³⁺ and Co(OH))RSH)²⁺ respectively. The mechanisms of the reactions are discussed.