Notes. Kinetics of reduction of tris(ethylenediamine)cobalt(III) by ethylenediamine-NNN′N′-tetra-acetatochromate(II)

Abstract

The kinetics of the reduction of [Co(en)₃]³⁺(en = ethylenediamine) by [Cr(edta)]^2– have been studied by stopped-flow methods under a variety of conditions. The reaction is first order in both reagents, with a second-order rate constant of 3.37 × 10³ dm³ mol^–1 s^–1 at 25 °C, pH 4.81, unit ionic strength and [edta]= 5 × 10^–2 mol dm^–3(edta = ethylenediamine-NNN′N′-tetra-acetate). Activation parameters (ΔH^‡= 56 kJ mol^–1, ΔS^‡= 12 J K^–1 mol^–1) are consistent with weak electrostatic attraction between the reactants. The rate shows an inverse dependence on the concentration of free edta, attributed to formation of an ion pair or hydrogen-bonded adduct with the oxidant. The pH dependence is a double sigmoid, consistent with two protonation equilibria in the reductant with pK_a values of 2.7 and 5.5, and the reaction rate increases with increasing numbers of negative donor groups in the complex.