Kinetics and mechanism of the two electron oxidation of cis-CrIII(dipy)2(H2O)2]3+ (dipy = 2,2′-dipyridyl) by periodate ion to cis-[CrV(dipy)2(O)2]+ in aqueous acidic solutions

Abstract

The kinetics of oxidation of cis-[CrIII(dipy)2(H2O)2]3+ (dipy = 2,2′-dipyridyl) by IO4 − has been studied in aqueous acidic solutions. The initial oxidation product is certainly not Cr(VI). The isolated solid product is consistent with the formula cis-[CrV(dipy)2(O)2]IO4. The Cr(V) product has been isolated and characterized by elemental analysis, IR and ESR spectroscopy. In the presence of a vast excess of [IO4 −], the reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, k obs, showed a very small change with increasing [IO4 −]. The dependence of k obs on [IO4 −] is consistent with Eq. (i). i $$ {\text{k}}_{\text{obs}} = {\text{a}}[{\text{IO}}_{4}^{\text{ - }} ]/({\text{b}} + {\text{c}}[{\text{IO}}_{4}^{\text{ - }} ]) $$ The pseudo-first-order rate constant, k obs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the reactive species. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are also reported.