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

Abstract

The kinetics and mechanism of oxidation of [Co^II HDTA]^4- (Where HDTA=Hexamethylenediamine tetraacetic acid} by periodate ion has been studied in aqueous acidic medium. The reactions has been investigated spectrophotometrically at λ_max = 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 [Co^II HDTA] ^4- and [IO₄^-] obeys inner sphere reaction pathway through the formation of long-lived intermediate complex which finally get converted into a corresponding [Co^IIIHDTA] ^3- complex as final reaction product. The experimental observations have shown that the reaction obey first- order dependence in [Co^II HDTA] ^4--. The variation of pseudo-first-order rate constants (k_obs) with[IO₄^-], keeping other reaction variables fixed at constant value was found to obey the rate law: k_obs=a[IO₄^-]2/b+c[IO₄^-], which is consistent with a three step mechanistic scheme. The values of k_obs are almost constant with increasing pH, which can be attributed to the reaction of deprotonated form of [Co^III HDTA] ^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^-1 mole^–1 respectively and support the proposed mechanistic scheme.