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

Abstract

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 \([{\rm IO}_4^-]\) and the complex concentration, and it obeys the following rate law: $$ d[{\rm Cr}^{\rm VI}]/dt=k_6 K_{4} K_{6} [{\rm IO}_4^-][{\rm Cr}^{\rm III}]_{\rm T}/\{([{\rm H}^+]+K_{4})+(K_{5}[{\rm H}+]+K_{6} K_{4})[{\rm IO}_4^- ]\}. $$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 \({\rm IO}_4^- \) to chromium(III). Thermodynamic activation parameters were calculated using the transition state theory equation. The mechanism of the oxidation of [CrIII(DPA)(OX)(H2O)] − (DPA = dipicolinate and OX = oxalate) by periodate under pseudo-first-order condition has been studied. The electron transfer proceeds through an innersphere mechanism via coordination of IO4 – to chromium(III).