Kinetics and mechanism of periodate oxidation of two ternary nitrilotriacetatochromium(III) complexes involving histidine and aspartate co-ligands

The oxidation of Cr^III(HNTA)(Hist)(H₂O)]⁻ and Cr^III(HNTA)(Asp)(H₂O)]⁻ (NTA = nitrilotriacetate, Hist = l-histidinate and Asp = dl-aspartate) by periodate in aqueous medium has been studied spectrophotometrically between 15.0 and 35.0 °C under pseudo-first-order conditions, IO₄ ⁻] ≫ complex]. The rate increases over the pH range 3.40–4.45 in both cases, but the two complexes give different rate laws. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of IO₄ ⁻ to chromium(III). A common mechanism for the oxidation of some chromium(III) complexes by periodate is proposed, and this is supported by an excellent isokinetic relationship between ΔH* and ΔS* values for these reactions.