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

The kinetics of oxidation of thiosulfate by the complexes [Ni(IV)L(2)(3)](2+) and [Ni(IV)L(2)](2+) (HL(3) = 6-amino-3-methyl-4-azahex-3-en-2-one oxime, H(2)L(2) = 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 x 10(-5) mol dm(-3), [S2O32-] = (1.0-10.0) x 10(-3) mol dm(-3), I = 0.20 mol dm(-3) (NaClO4) at 20 degrees C. A monophasic two-electron transfer was found in the regions 3.0 less than or equal to pH less than or equal to 5.0 and 2.5 less than or equal to pH less than or equal to 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+)][S2O32-] 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 (k(12)) obtained through Marcus cross-reaction relations can be considered as a proof of such delineation.