Activation of copper(II) ammine complexes by molecular oxygen for the oxidation of thiosulphate ions

Abstract

The stoicheiometry and kinetics of reaction of aqueous-ammonia (0·1–1.0M) solutions of copper(II) ions with thiosulphate ion in the presence of oxygen have been examined. The amount of oxygen consumption and the relative amounts of the final sulphur products, namely trithionate and sulphate ions, are dependent on the initial [S₂O₃]^2– concentration and pH. The detailed kinetics of the reaction at pH 11·2 suggest mechanisms in which O₂ becomes axially associated with amminethiosulphatocopper(II) species. In this role O₂ assists in electron transfer between [S₂O₃]^2– and Cu^II. The most active species for [S₃O₆]^2– formation is a tetra-amminecopper(II) complex having one axial [S₂O₃]^2– and one axial O₂ ligand. A triamminecopper(II) complex, having both axial and equatorial [S₂O₃]^2– ligands as well as an axial O₂, is suggested as the reactive intermediate for [SO₄]^2–formation. The role of the intermediate Cu^II species is one of complexing both oxidant and reductant.,in that it provides a mechanism for electron transfer and allows O₂ to interact via an ionic mechanism.