Additional Aspects of Complexation of Gold(I) with Thiomalate

Abstract

Some equilibria involving gold(I) thiomalate (mercaptosuccinate, TM) complexes have been studied in the aqueous solution at 25 °C and I = 0.2 mol·L−1 (NaCl). In the acidic region, the oxidation of TM by $$ {\text{AuCl}}_{4}^{ - } $$ proceeds with the formation of sulfinic acid, and gold(III) is reduced to gold(I). The interaction of gold(I) with TM at nTM/nAu ≤ 1 leads to the formation of highly stable cyclic polymeric complexes $$ {\text{Au}}_{m} \left( {\text{TM}} \right)_{m}^{*} $$ with various degrees of protonation depending on pH. In general, the results agree with the tetrameric form of this complex proposed in the literature. At nTM/nAu > 1, the processes of opening the cyclic structure, depolymerization and the formation of $$ {\text{Au}}\left( {\text{TM}} \right)_{2}^{*} $$ occur: $$ {\text{Au}}_{4} ( {\text{TM)}}_{4}^{8 - } + {\text{TM}}^{3 - } \rightleftharpoons {\text{Au}}_{ 4} ( {\text{TM)}}_{5}^{11 - } $$ , log10 K45 = 10.1 ± 0.5; 0.25 $$ {\text{Au}}_{4} ( {\text{TM)}}_{4}^{8 - } + {\text{TM}}^{3 - } \rightleftharpoons {\text{Au(TM)}}_{2}^{5 - } $$ , log10 K12 = 4.9 ± 0.2. The standard potential of $$ {\text{Au(TM)}}_{2}^{5 - } $$ is $$ E_{1/0}^{ \circ } = -0. 2 5 5\pm 0.0 30{\text{ V}} $$ . The numerous protonation processes of complexes at pH < 7 were described with the use of effective functions.