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

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 lambda(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 0 degrees 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-III HDTA](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-II 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, Delta H-# = 28.69 kJ mole(-1); Delta S-# = - 481.13 J K-1 mole(-1) respectively and support the proposed mechanistic scheme.