Kinetic and mechanism of oxidation of chromium (III)-nicotinate complex by periodate

Abstract

The kinetics of oxidation of [Cr III (nic)(H2O)2]+(nic = nicotinate) by IO4 − to Cr(VI) have been studied spectrophotometrically. The reactions exhibited biphasic kinetic behavior. This has been interpreted in terms of consecutive two step reactions. The first faster step involved the oxidation of the Cr (III) complex to a Cr (IV) species. The second slower reaction involved the oxidation of Cr (IV) species to Cr (V). Both steps exhibited first order dependence on the initial [Cr (III)] as well as first order dependence on [IO4 −]. The rates of both processes increased with increasing pH due to the deprotonation equilibria of the complex. The experimental rate law was consistent with a mechanism in which the deprotonated form [Cr III (nic)(H2O)(OH)] was more reactive than the conjugated acid. It was proposed that electron transfer proceeds through an inner sphere mechanism via coordination of IO4 to chromium (III). Thermodynamic activation parameters were calculated using the transition state theory equation.