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Kinetics and mechanism of oxidation of a ternary nitrilotriacetatochromium(III) complexes involving L-histidine and DL-aspartic as a secondary ligands by periodate

Hassan A.Ewais, Mohamed A.Habib

The oxidation of[CrIII(NTA)(Hist)(H2O)]- and[CrIII(NTA)(Asp)(H2O)]- (NTA = nitrilotriacetic acid, Hist = L-histidine and Asp=DL-aspartic acid) by periodate in aqueous medium has been studied spectrophotometrically over 15.0-35.00C range. The reaction is first order with respect to both[IO4 -] and the complex concentration, and the rate increases over the pH range 3.40- 4.45 in both cases. The experimental rate lawis consistentwith a mechanism in which both the deprotonated forms,[CrIII(NTA)(Hist)(OH)]2- and[CrIII(NTA)(Asp)(OH)]2- are significantly more reactive than the conjugate acids. It is proposed that electron transfer proceeds through an innersphere mechanism via coordination of IO4 - to chromium(III). A common mechanismfor the oxidation of some chromium(III) complexes by periodate is proposed, and this is supported by an excellent isokinetic relationship between H* and S* values for these reactions.