Oxidation of the thiosulfate ion with a (µ-oxo)diiron(III) complex: kinetic evidence for self reaction of the thiosulfate radical ion

Abstract

In aqueous Hphen⁺–phen buffer media (pH 3.9–5.2), the complex ion [Fe^III₂(µ-O)(phen)₄(H₂O)₂]⁴⁺ 1 (phen = 1,10-phenanthroline) has been shown to oxidise the thiosulfate ion to the tetrathionate ion. In the presence of excess phenanthroline, 1 itself was reduced to [Fe(phen)₃]²⁺ 3²⁺, an inhibitor for the reaction. According to the proposed mechanism, an inner-sphere, one-electron reduction of 1 by S₂O₃^2– generates [Fe(phen)₃]³⁺ 3³⁺, [Fe(phen)₃]²⁺ 3²⁺ and the thiosulfate radical ion S₂O₃^–; 3³⁺ is then further reduced to 3²⁺ by another S₂O₃^2– ion. The radical ion S₂O₃^– either re-oxidises 3²⁺ to 3³⁺ and thus inhibits the reaction, or produces the tetrathionate ion through a self reaction. Kinetic activity of 1 swamps that for its hydrolysed derivatives.