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 Co^IIL(H₂O)]^2-n L = trimethylenediaminetetraaceticacid (TMDTA)] and ethylene glycol bis(2-aminoethyl ether) N,N,N’,N’-tetraaceticacid (EGTA) have been studied spectrophotometrically by following an increase in absorbance at λ_max = 550 nm in acetate buffer medium as a function of pH, ionic strength, temperature, various concentration of periodate and Co^IIL(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 Co^IIIL(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) Co^IIL(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 Co^IIL(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 Co^IITMDTA(H₂O)]^2?-IO₄]^? system but it decreases considerably in case of Co^IIEGTA(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.