Abstract
The kinetics of oxidation of [CrIIIcdta(H2O)]- and [CrIIIdtpa(H2O)]2- (where cdta = trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetate and dtpa = diethylenetriaminepentaacetate) by periodate ion has been studied in aqueous solutions. The oxidation of these complexes was carried out in the pH range 5.527.44 for the [CrIIIcdta(H2O)]- complex and the pH range 5.568.56 for the [CrIIIdtpa(H2O)]2- complex. The reaction exhibited an uncommon second-order dependence on [CrIIIL(H2O)]n (L = cdta or dtpa and n=-1 or -2, respectively) and a first-order dependence on [IO-4]. At fixed reaction conditions, the reaction rate is described by Eq. (i). The third-order rate constant, k3, varied with [H+] according to Eq. (ii). i ii A mechanism in which simultaneous one-electron transfer from two [CrIIIL(OH)]n-1 ions to I(VII) is proposed. The two [CrIIIL(OH)]n-1 ions are bridged to I(VII) via the hydroxo group. Periodate ion is known to undergo rapid substitution or expansion of its coordination number from four to six. The activation parameters Delta H* and Delta S* were calculated using the Eyring equation. The relatively high negative values of Delta S* are consistent with an associative process preceding electron transfer. (c) 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 729735, 2012