The Kinetics and Mechanism of Oxidation of Triethylenetetraaminehexaacetatocobaltate(II) Complex by Periodate Ion in AqueousMedium

Abstract

The kinetics and mechanism of oxidation of [Co(II)TTHA]4- (where TTHA = Triehylenetetraaminehexaaceticacid) by periodate ion has been studied in aqueous acidic medium. The reaction has been investigated spectrophotometrically at λmn = 550 nm under pseudo first- order condition by taking large excess of oxidant, [IO4-] at pH = 4.0 + 0.02, 1 = 0.1 M (CH3COONa + Na NO3) and at three different temperatures viz 150 C, 200 C and 250 C. The electron transfer reaction between [Co(II)TTHA]4- and , [IO4-] obeys inner- sphere reaction pathway through the formation of long lived intermediate complex which eventually get converted into a corresponding [Co(III)TTHA]3- complex as final reaction product. The experimental observations have shown that the reaction exhibit first- order dependence in [Co(II)TTHA]4- . The variation of pseudo- first- order rate constant (Kabc) with [IO4-] keeping other reaction variables fixed at constant value is found to obey the following empirical equation. Kabc = a[IO4-]2/b + c[IO4-] This rate law is consistent with a three step mechanistic scheme. The values of (Kabc) are almost invariant with increasing pH and attributed due to the reaction of deprotonated form of [Co(II)TTHA]4- complex and [IO4-] in the whole pH region. Eyring’s equations has been used to calculate the activation parameters and found to be ∆H* = 16.63 KJ mole-1 ; ∆S* = -214.00 JK-1 mole-1. These values are consistent with three step mechanistic scheme as proposed.