Kinetics and mechanism of oxidation of cis-diaquabis(glycinato)chromium(III) by periodate ion in aqueous solutions

Abstract

The kinetics of oxidation of cis-[CrIII(gly)2(H2O)2]+ (gly = glycinate) by $$ {\text{IO}}_{ 4}^{ - } $$ has been studied in aqueous solutions. The reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, k obs, showed a small change with increasing $$ \left[ {{\text{IO}}_{ 4}^{ - } } \right] $$ . 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. The reaction has been found to obey the following rate law: $$ {\text{Rate}} = 2k^{\text{et}} K_{ 3} K_{ 4} \left[ {{\text{Cr}}\left( {\text{III}} \right)} \right]_{t} \left[ {{\text{IO}}_{ 4}^{ - } } \right]/\left\{ {\left[ {{\text{H}}^{ + } } \right] + K_{ 3} + K_{ 3} K_{ 4} \left[ {{\text{IO}}_{ 4}^{ - } } \right]} \right\} $$ . Values of the intramolecular electron transfer constant, k et, the first deprotonation constant of cis-[CrIII(gly)2(H2O)2]+, K 3 and the equilibrium formation constant between cis-[CrIII(gly)2(H2O)(OH)] and $$ {\text{IO}}_{ 4}^{ - } $$ , K 4, have been determined. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are reported.