Reduction of the Gold(III) Complex Ion, [AuCl3OH]-, By N-Methylthiourea, N, N’–Dimethylthiourea and N, N’- Diethylthiourea in Aqueous Acid: Kinetic, Thermodynamic And Mechanistic Studies

Abstract

The kinetic, thermodynamic and mechanistic studies of the reduction of the gold(III) complex ion, [AuCl3OH]- by N-methylthiourea (MTU), N, N’- dimethylthiourea (DMTU) and N, N’– diethylthiourea (DETU) were studied spectrophotometrically in aqueous acid. Stoichiometry revealed that two moles of N-methylthiourea, N,N’–dimethylthiourea and N,N’- diethylthiourea each were oxidised by one mole of gold(III) ion. The rate showed a first order dependence on [AuCl3OH]- and half-order on [reductants] with one and half order overall. The reactions were biphasic revealing an initial substitution step and a final electron transfer phase. Addition of acid within the range 5.0 x10-4 - 1.0 x 10-2 mol dm-3 led to decrease in the reaction rate. The rate equation at constant [H+] and ionic strength is where = 0.335±0.09 ((MTU), 0.389±0.09 (DMTU and 0.408±0.11 (DETU) for the substitution process, 0.044±0.003 (MTU), 0.058±0.001 (DMTU) and 0.059±0.002 (DETU) for the electron transfer reaction, Varying ionic strength from 1.0 x 10-2 - 1.0 x10-1 mol dm-3 had no significant effect on the rate of the reaction. Added anion and cation as well as change in the dielectric constant of the reactions media had no effect on the reactions rates. Presence of free radical was detected in the course of the reactions. Entropies of activation (JK-1 mol-1) were -189.65, -215.82 and -214.74 for MTU, DMTU and DETU respectively while activation enthalpies (kJ mol-1) were 7.329, 8.424 and 3.837 for MTU, DMTU and DETU. For all the reactions, Au(I) was found to be the product of [AuCl3(OH)]- reduction while disulphide was obtained for reductants oxidation. Michaelis-Menten’s type plot showed appreciable positive intercepts for these reactions. Mechanisms consistent with experimental data have been proposed.