Kinetics and mechanism of the formation and oxidation of iminodiacetato–chromium(III) complex

Abstract The kinetics of formation of chromium(III)–iminodiacetic acid complex have been studied spectrophotometrically in the temperature range 35–55 °C. The reaction is first order with respect to chromium(III) and the rate of reaction increases with increasing of pH. The kinetics of formation obeys the rate law: $$ \mathrm{Rate}={k}_{\mathrm{c}}{K}_{\mathrmósc}}{\left[{\mathrm{Cr}}^{\mathrmÍII}}{\left({\mathrm{H}}_2\mathrmÓ}\right)}_5\left(\mathrmÓH}\right)\right]}_T^{2+}\left[\mathrmÍDA}\right]/\left(1+{K}_{\mathrmósc}}\left[\mathrmÍDA}\right]\right). $$ While the kinetics of oxidation of CrIII(IDA)2(H2O)2]− by N-bromosuccinimide (NBS) in aqueous solution to chromium(VI) were carried out and obey the following equation: $$ \mathrm{d}\left[{\mathrm{Cr}}^{\mathrm{VI}}\right]/\mathrm{d}t=\left({k}_4{K}_2+{k}_5{K}_3{K}_1/\left[{\mathrm{H}}^{+}\right]\right){\left[{\mathrm{Cr}}^{\mathrmÍII}}{\left(\mathrmÍDA}\right)}_2{\left({\mathrm{H}}_2\mathrmÓ}\right)}_2\right]}^{-}\left[\mathrm{NBS}\right]. $$ Thermodynamic activation parameters have been calculated. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of NBS to chromium(III).