Complexation of a Nickel(III) Macrocyclic Complex by Sulfate Ion. A Pulse Radiolytic Study

The kinetics of oxidation of Ni₁₁[meso-Me₆[14]aneN₄] (NiL²⁺) to Ni(III) by OH radical and subsequent complexation by sulfate ion have been studied in acidic aqueous media at 22°C and μ=0.01-0.3 M by the pulse radiolytic technique. Results indicate that NiL3+hydrolyzes with a pK+3.7±0.2 but that, at least up to pH 5.2, substitution by SO₄ ^2- occurs only on the unhydrolyzed species with a rate constant of 1X10₆ M^-1 s^-1 at μ=0.03 M. The equilibrium constant for sulfate complexation is 3.0X10₄ M^-1 at μ=0.03 M and the product of the reaction is identical with the stable NiLSO₄+species produced by the electrochemical oxidation of NiL2+in sulfate media. The results demonstrate the stabilization of NiIII by sulfate complexation.