By Ugwuanyi Kenneth Okeke
The kinetics and mechanisms of the redox reactions of oxidant, -adipato-di(N,N/-bis(salicylidene)ethylenediaminatoiron(III),[(Fe-salen)2adi],hereafter denoted as Fe2adi, with the thiols L-cysteine (LSH), thiourea (USH), thioglycolicacid(GSH),2–mercaptobenzothiazole (BTSH) and benzylmercaptan (BSH) have been studied spectrophotometrically at 495 nm in aqueous perchloric acid, I=0.01mol dm-3 (NaClO4) and at 29 1oC. The stoichiometry of 1:1 was obtained for Fe2adi – LSH, Fe2adi – GSH and Fe2adi –BTSH systems while 1:2 was obtained for Fe2adi – USH and Fe2adi–BSH systems. Under pseudo – first order conditions of a large excess of the reductants, pseudo–first order rate constants increased with increase in concentrations of the thiols (LSH, USH, GSH and BTSH), but decrease in pseudo – first order rate constant was observed as the concentration of BSH increased. The second order rate constants k2 were fairly same for all the five systems with values. 0.009 0.003 dm3 mol-1 s-1, 0.063 0.01dm3 mol-1 s-1, 0.038 0.005 dm3 mol-1 s-1, 0.011 0.001 dm3 mol-1 s-1. and. 0.0015 0.0035 dm3 mol-1 s-1 for LSH, USH, GSH, BTSH and BSH respectively. The rates of reaction were directly dependent on acid concentrations for the five systems. The overall rate equation for the reactions can be given as [Fe2adi] [reductant] [ ] for Fe2adi–LSH, Fe2adi – GSH, and Fe2adi - BTSH systems where a and b are the intercept and slope respectively, [Fe2adi] [USH]2 for Fe2adi–USH, while the rate equation, was for Fe2adi –BSH system is [Fe2adi] [BSH] [ ]2. The values of a and b obtained for the reaction of Fe2adi with the thiols are given as LSH (a = 0.3 x 10-4 dm3mol-1 s-1 and b = 1.23 x 10-3 dm6 mol-2 s-1), USH (a=6.2x10-4 dm3mol-1s-1 and b = 6.4 x10-3 dm6mol-2s-1), GSH (a = 1.9 x 10-4 dm3mol-1s-1 and b = 8.18 x 10-2 dm6mol-2 s-1), BTSH (a = 0.51 x 10-5 dm3mol-1s-1 and b = 2.0 x 10-3 dm6mol-2s-1), BSH (b =8.0 x 10-3 dm6mol-2 s-1). The rates of reactions was observed to decrease with the increase in ionic strength of the medium for the five systems under study. Addition of magnesium and acetate ions in small amount did not affect the rates of reactions for the redox reactions of Fe2adi with LSH, USH, GSH and BTSH, but decrease in rates of reaction was observed for that of BSH. The rates of reaction were not affected by the decrease in dielectric constant D for the reaction of Fe2adi with LSH, GSH and BTSH but it was enhanced for USH and BSH under the same conditions. Furthermore, the activation parameters, H# and S# were also determined for the five systems; the values being 89.99kJmol-1 and -30195Jk-1 mol-1 for Fe2adi-LSH system, 20.68kJmol-1 and -197.60Jk-1mol-1 for Fe2adi-USH system, 27.38kJmol-1 and-303.48JK-1 mol-1 for Fe2adi-BTSH system and 12.83kJmol-1 and -313.82JK-1mol-1 for Fe2adi-BSH system. Michaelis-Menten plot of 1/kobs versus 1/[reductants] were linear with intercepts for the redox reactions of Fe2adi with LSH, GSH, BTSH, USH and BSH. On the basis of the results obtained above, the reactions have been proposed to follow the inner-sphere mechanism.