Kinetics and mechanism of oxidation of ethylenediamine by dihydroxydiperiodatonickelate(IV) complex ion in alkaline medium

The kinetics of oxidation of ethylenediamine(en) by dihydroxydiperiodatonickelate (IV) complex (DDN) was studied spectrophotometrically in aqueous alkaline medium([OH-]/(mol . L-1) = 0. 050- 0. 40) at 20-35 degrees C. The reaction rate showed first-order dependence in oxidant and positive fractional order dependence in ethylenediamine. The pseudo first-order ([en]>>[DDN])> rate constant k(obs) increases with the increase of [OH-] and decreases with the increase of [IO4-], which implies that before the rate-determining step there is a preequilibrium in which one of the periodate ligands of the oxidant is deprotonated and the other is dissociated, and dihydroxymonoperiodate (IV) complex(DMN) is the reactive species. In addition, no salt effect and no free radical was observed, which indicates that an innersphere one-step two-electron transfer mechanism without free radical intermediate may be in operation. Based on the experimental results a mechanism involving a preequilibrium of an adduct formation between the [Ni(OH)(2)(HIO6)](2-) and ethylenediamine has been proposed. The rare equation derived from the mechanism can be used to explain all the experimental results satisfactorily and the preequilibrium constants, rate constant of the rate-determining step together with the activation parameters were evaluated.