Kinetic and Mechanistic Studies of Oxidation of Amine-N-Polycarboxylates Complexes of Cobalt(II) by Periodate Ions in Aqueous Medium

The kinetics and mechanism of interaction of periodate ion with [CoIIL(H2O)]2-n [L = trimethylenediaminetetraaceticacid (TMDTA)] and ethylene glycol bis(2-aminoethyl ether) N,N,Nʹ,Nʹ- tetraaceticacid (EGTA) have been studied spectrophotometrically by following an increase in absorbance at lmax = 550 nm in acetate buffer medium as a function of pH, ionic strength, temperature, various concentration of periodate and [CoIIL(H2O)]2-n under pseudo-first order conditions. The experimental observations have revealed that the intermediates having sufficiently high half life are produced during the course of both the reactions which finally get converted into a corresponding [CoIIIL(H2O)]3-n complexes as a final reaction product. The reaction is found to obey the general rate law Rate = (k2 [IO4 -] + k3 [IO4 -]2) [CoIIL(H2O)]2-n. This rate law is consistent with a four step mechanistic scheme (vide supra) where electron transfer proceeds through an inner sphere complex formation. The value of rate constant k2 is independent of pH over the entire pH range which suggest that unprotonated form of [CoIIL(H2O)]2-n is the only predominant species. The value of k2 is invariant to ionic strength variation in both the systems. The value of k3 is also found to be almost invariant to ionic strength in case of [CoIITMDTA(H2O)]2--[IO4]- system but it decreases considerably in case of [CoIIEGTA(H2O)]2--[IO4]- system with the corresponding decrease in ionic strength. The activation parameters have been computed and given in support of proposed mechanistic scheme