Kinetics and mechanism of the one‐step, two‐electron oxidation of diaquadichloro(1,10‐phenanthroline)chromium(III) chloride dihydrate by periodate to chromium(V) in aqueous acidic solutions

Abstract

AbstractThe kinetics of oxidation of [CrIIICl2(phen)(H2O)2]Cl⋅2H2O (phen = 1,10‐phenanthroline) by periodate {I(VII)} have been studied in aqueous acidic solutions. The oxidation product has been isolated and characterized by elemental analysis, IR, and ESR as [CrVCl2(phen)(O)]Cl⋅1.5H2O. Under pseudo–first‐order conditions, the reaction showed first‐order dependence on the initial Cr(III) complex concentration. At fixed reaction conditions, the dependence of kobs on [I(VII)] showed saturation kinetics. The rate constant, kobs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the most reactive species. A plot of kobs([H+] + K1) versus [H+] is linear with an intercept, indicating that the reaction proceeds via two parallel pathways. One pathway is independent of [H+] and the other on [H+]–1. Since water is known to be a very weak bridging ligand, the pathway involving the aqua form of the complex, almost certainly, is bridged to periodate via the chloro ligand of the chromium(III) complex. The thermodynamic parameters of the processes involved are reported. A one‐step two‐electron transfer via an inner‐sphere mechanism has been assigned to the two pathways based on the form of the rate law