Gold(I) cysteinate complexes in aqueous solutions

Resumo

Equilibria between gold(I) cysteinate complexes in aqueous solutions are studied by pH-metry, solubility measurements, and potentiometric measurements using a gold electrode at 25°C and I = 0.2 M (NaCl). The stepwise protonation constants logK_iH of cysteinate H_{i – 1}Cys^{i – 3} + H⁺ = H_iCys^{i – 2} are as follows: 10.10 (i = 1), 8.12 (i = 2), and 1.90 (i = 3). Protonation of cysteinate complexes is studied: Au(Cys)₂³⁻ + H⁺ = Au(HCys)(Cys)^2–, logK_{1H Au} = 9.62 and Au(HCys)(Cys)^2– + H⁺ = Au(HCys)₂⁻, logK_2HAu = 9.01. The constant K_S of the heterogeneous equilibrium Au(HCys)_s + HCys^– = Au(HCys)₂⁻ is 3.3. The standard potential of the Au(Cys)₂³⁻ complex is E_1/0⁰ = –0.172 ± 0.011 V. The stability of the Au(HCys)₂⁻ complex is close to that of Au(HS)₂⁻. The constant of substitution Au(HCys)₂⁻ + SO₃²⁻ = Au(HCys)SO₃²⁻ + HCys^– is determined to be logK_11H = –1.3.