Concurrent two one‐electron transfer in the oxidation of chromium(III) complexes with trans‐1,2‐diaminocyclohexane‐N,N,N′,N′‐tetraacetate and diethylenetriaminepentaacetate ligands by periodate ion

The kinetics of oxidation of Cr^IIIcdta(H₂O)]^? and Cr^IIIdtpa(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 5.52–7.44 for the Cr^IIIcdta(H₂O)]^? complex and the pH range 5.56–8.56 for the Cr^IIIdtpa(H₂O)]^2? complex. The reaction exhibited an uncommon second‐order dependence on Cr^IIIL(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 Cr^IIIL(OH)]^n?1 ions to I(VII) is proposed. The two Cr^IIIL(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 ΔH* and ΔS* were calculated using the Eyring equation. The relatively high negative values of ΔS* are consistent with an associative process preceding electron transfer. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 729–735, 2012