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

The kinetics of oxidation of ethylenediamine(en) by dihydroxydiperiodatonic kelate (IV) complex (DDN) was studied spectrophotometrically in aqueous alk aline medium([OH-]/(mol . L-1) = 0. 050- 0. 40) at 20-35 degrees C. The rea ction 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 wi th the increase of [IO4-], which implies that before the rate-determining s tep there is a preequilibrium in which one of the periodate ligands of the oxidant is deprotonated and the other is dissociated, and dihydroxymonoperi odate (IV) complex(DMN) is the reactive species. In addition, no salt effec t 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-) an d ethylenediamine has been proposed. The rare equation derived from the mec hanism can be used to explain all the experimental results satisfactorily a nd the preequilibrium constants, rate constant of the rate-determining step together with the activation parameters were evaluated.