Kinetic studies on the oxidation of sulfur(IV) by nickel(IV) oxime–lmine complexes

Abstract

The oxidation of sulfite by the two nickel(IV) oxime–imine complexes [Ni^IVL¹₂₊]² and [Ni^IVL²]²⁺(HL¹= 6-amino-3-methyl-4-azahex-3-en-2-one oxime and H₂L²= 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,1 5-dione dioxime) has been investigated in aqueous medium at 30 °C over the range pH 2.0–8.0. Single-step two-electron transfer reactions were encountered in the regions 2.0 ⩽ pH ⩽ 5.50 for [Ni^IVL¹₂]²⁺ and 2.0 ⩽ pH ⩽ 4.25 for [Ni^IVL²]²⁺ reductions. A distinct biphasic process with faster initial one-electron reduction of Ni^IV to Ni^III followed by a slower conversion of Ni^III into Ni^II was observed above pH 5.75 and 4.50 for the respective complexes. Attempts were made to evaluate the reactivity of all the reacting species of the complexes as well as of sulfur(IV) by considering suitable pH regions. The stoichiometries of the reactions ([S^IV]:[Ni^IV]) were found to vary within the region ≈2.0–1.0 depending upon the mole ratios of the reactants. At pH 4.50 with an excess of complex over the reductant the ratio [S^IV]:[Ni^IV] is less than 2:1 for both complexes giving a mixture of dithionate and sulfate as the oxidation products of sulfite. However, with an excess of sulfite over the complex this ratio reaches a limiting value of ≈2:1 at [S^IV]:[Ni^IV] > 8 giving dithionate as the major oxidation product. The observed stoichiometric ratios for both complexes are in accord with the distribution of the sulfate and dithionate products. Attempts were made to correlate the experimentally observed rate constants with those obtained from Marcus cross-relation calculations. Calculated rate constants for the oxidation of SO₃^2– by various nickel-(IV) and -(III) species were found to be one to two orders of magnitude lower than the corresponding experimental values. The oxidation of SO₂·H₂O and HSO₃^– is proposed to proceed through the formation of a hydrogen-bonded adduct.