Kinetics and mechanism of oxidation of the binary and ternary complexes of chromium(III) involving inosine and glycine by N -bromosuccinimide

Abstract

The kinetics of oxidation of the chromium(III) complexes, [Cr(Ino)(H2O)5]3+ and [Cr(Ino)(Gly)(H2O)3]2+ (Ino = Inosine and Gly = Glycine) involving a ligands of biological significance by N-bromosuccinimide (NBS) in aqueous solution to chromium(VI) have been studied spectrophotometrically over the 25–45°C range. The reaction is first order with respect to both [NBS] and [Cr], and increases with pH over the 6.64–7.73 range in both cases. The experimental rate law is consistent with a mechanism in which the hydroxy complexes [Cr(Ino)(H2O)4(OH)]2+ and [Cr(Ino)(Gly)(H2O)2(OH)]+ are significantly more reactive than their conjugate acids. The value of the intramolecular electron transfer rate constant, k 1, for the oxidation of the [Cr(Ino)(H2O)5]3+ (6.90 × 10−4 s−1) is lower than the value of k 2 (9.66 × 10−2 s−1) for the oxidation of [Cr(Ino)(Gly)(H2O)2]2+ at 35°C and I = 0.2 mol dm−3. The activation parameters have been calculated. Electron transfer apparently takes place via an inner-sphere mechanism. †ed from the MSc thesis of Mohamed A. Nagdy.