Electron transfer from thiosulfate to nickel(IV) oxime imine complexes in aqueous solution

Abstract

The kinetics of oxidation of thiosulfate by the complexes [Ni^IVL³₂]²⁺ and [Ni^IVL²]²⁺(HL³= 6-amino-3-methyl-4-azahex-3-en-2-one oxime, H₂L²= 3,14-dimethyl-4,7,10,13-tetraazahexadeca-3,13-diene-2,15-dione dioxime) was studied spectrophotometrically under pseudo-first-order conditions with an excess of thiosulfate in the range pH 2.50–8.0 with [Ni^IV]= 5.0 × 10^–5 mol dm^–3, [S₂O₃^2–]=(1.0–10.0)× 10^–3 mol dm^–3, I= 0.20 mol dm^–3(NaClO₄) at 20 °C. A monophasic two-electron transfer was found in the regions 3.0 ⩽ pH ⩽ 5.0 and 2.5 ⩽ pH ⩽ 4.0 for reduction of the two complexes respectively. Above these pH regions both reactions showed biphasic decay with an initial faster step Ni^IV→ Ni^III and second slower step Ni^III→ Ni^II, and the general rate law is –d[Ni^IV(L_x)²⁺]/dt=k_obs[Ni^IV(L_x)²⁺]= 2nk[Ni^IV(L_x)²⁺][S₂O₃^2–] where 2n is a stoichiometric factor. All the kinetic parameters have been evaluated by choosing suitable pH regions. An additional protonation of the nickel(IV) complexes (pK_a= 4.91 and 4.28 respectively) has been encountered and a realistic interpretation is achieved by considering a weak acid–base interaction arising out of outer-sphere protonation of the [Ni^IV(L_x)]²⁺ complexes. All the reaction steps have been scrutinised. The reduction [Ni^IV(L_x)]²⁺→[Ni^III(L_x)]⁺ is assumed to follow an outer-sphere mechanism whereas [Ni^III(L_x)]⁺→[Ni^II(L_x)] follows an inner-sphere route. The calculated electron-transfer rate constant (k₁₂) obtained through Marcus cross-reaction relations can be considered as a proof of such delineation.