Mechanism of electron transfer reaction of ternary dipicolinatochromium(III) complex involving oxalate as secondary ligand

Mechanism of the oxidation of [CrIII(DPA)(OX)(H2O)] −  (DPA = dipicolinate and OX = oxalate) by periodate in aqueous acidic medium has been studied spectrophotometrically over the pH range of 4.45–5.57 at different temperatures. The reaction is first order with respect to both \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$[{\rm IO}_4^-]$\end{document} and the complex concentration, and it obeys the following rate law: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ d[{\rm Cr}^{\rm VI}]/dt=k_6 K_{4} K_{6} [{\rm IO}_4^-][{\rm Cr}^{\rm III}]_{\rm T}/\{([{\rm H}^+]+K_{4})+(K_{5}[{\rm H}+]+K_{6} K_{4})[{\rm IO}_4^- ]\}. $$\end{document}The rate of the reaction increases with increasing pH due to the deprotonation equilibria of the complex. The experimental rate law is consistent with a mechanism in which the deprotonated form [CrIII(DPA)(OX)(OH)]2 −  is more reactive than the conjugated acid. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\rm IO}_4^- $\end{document} to chromium(III). Thermodynamic activation parameters were calculated using the transition state theory equation.