Metal-ion oxidations in solution. Part IX. Characterisation of the intermediate formed in the oxidation of thiomalic acid by iron(III) ions

Abstract

A spectrophotometric stopped-flow study of the redox reaction between iron(III) ions and thiomalic acid (2-mer-captosuccinic acid) has shown the existence of a transient intermediate which is believed to be an iron(III) complex in which thiomalate acts as a bidentate chelate ligand. The visible absorption spectrum of this complex has a maximum at 610–620 nm, values of the absorption coefficient and equilibrium constant being 350 ± 20 l mol^–1 cm^–1 and 2·3 ± 0·2 mol l^–1 respectively at 20 °C at I= 1·0M. Variation of the hydrogen-ion concentration has enabled the derivation of the approximate rate constants at 20 °C of 1 l mol^–1 s^–1 for complex formation between Fe³⁺ and thiomalic acid, and 10⁴ and 10⁶ l mol^–1 s^–1 for the reactions of FeOH²⁺ with the undissociated and monodissociated ligand respectively. The proposed mechanism and derived values for the rate constants are consistent with the view that the rate-determining step involves loss of a water molecule co-ordinated to Fe^III. Thermodynamic data for the associated equilibria have been evaluated from temperature-dependence studies but the complexity of the rate law, and the limited range of conditions which could be studied without making allowance for the subsequent redox reaction, have not permitted the assignment of activation parameters to the rate data to be made.