Kinetics and Mechanism of Oxidation of Hexamethylenediaminetetraacetatocobaltate(II) Complex by Periodate Ion in Aqueous Medium

Abstract

The kinetics and mechanism of oxidation of [CoIIHDTA]4-(Where HDTA=Hexamethylenediamine tetraacetic acid} by periodate ion has been studied in aqueous acidic medium. The reactions has been investigated spectrophotometrically at λmax= 580 nm under pseudo- first -order condition by taking large excess of oxidant [IO4] at pH = 4.0±0.02, I = 0.1 M (CH3COONa + NaNO3).and temperature = 30± 0.1°C The electron transfer reaction between [CoIIHDTA]4-and [IO4-] obeys inner sphere reaction pathway through the formation of long-lived intermediate complex which finally get converted into a corresponding [CoIIIHDTA]3-complex as final reaction product. The experimental observations have shown that the reaction obey first- order dependence in [CoIIHDTA]4--. The variation of pseudo-first-order rate constants (kobs) with[IO4-], keeping other reaction variables fixed at constant value was found to obey the rate law: kobs=a[IO4-]2/b+c[IO4-], which is consistent with a three step mechanistic scheme. The values of kobsare almost constant with increasing pH, which can be attributed to the reaction of deprotonated form of [CoIIIHDTA]4-complex only, in the entire pH region. Eyring’s equation has been used to calculate the thermal or activation parameters and found to be, ΔH#= 28.69 kJ mole–1; ΔS#= – 481.13 J K-1mole–1respectively and support the proposed mechanistic scheme.