The speciation of gold in aqueous solution: A theoretical study

Structures, energetics, and spectra have been calculated for a number of gold chloride, sulfide, and other complexes which may be present when Au or gold sulfides dissolve in aqueous solutions. Au(I) species with one ligand, such as Au(SH) or Au(OH), are found to be strongly coordinated with H2O, while Au(I) species with two ligands, such as Au(SH)(2)(-1), are coordinated to H2O only by hydrogen bonds. Negatively charged species with three ligands, such as Au(SH)(2)OH-2, spontaneously dissociate to two coordinate species plus a free ligand. Au(SH) combines with H2S to form the species Au(SH)(H2S)rather than its unstable three-coordinate isomer HAu(SH)2, but the interaction of Au(SH) with H2S is only slightly stronger than that with H2O. In aqueous solutions at low pH Au(SH)(H2O) will therefore predominate due to the high concentration of water. In near-neutral solutions the predominant species will be Au(SH)(2)(-1). In high pH solutions the calculations indicate Au(SH)(OH)(-1) to be the stable species, rather than dimers like Au2S22-. AuS3-1 is calculated to be stable with respect to Au(SH)(2)(-1), and this and other disulfide species will predominate in S-saturated solutions. In the absence of sulfide and chloride the stable species is calculated to be Au(OH)(H2O). Au(H2O)(2)(+) becomes stable only at very low pH. Au(III) complexes with chloride, hydroxide, and other ligands have also been studied and their calculated vibrational and visible/UV spectral properties are in reasonable agreement with experiment. The replacement of Cl- in AuCl4-1 by OH- is calculated to be exothermic. Although the potential energy surface for various mechanisms of Cl- exchange has not been fully explored, we have located an intermediate of formula AuCl3(H2O) which lies about 8 kcal/mol above the reactants. The energy of this intermediate is consistent with the experimentally determined value for the activation energy of the Cl- replacement reaction. To help develop the capabilities of Au Mossbauer spectroscopy for the determination of speciation we have calculated electric field gradients and electron densities at the Au nucleus for all the Au(I) and Au(III) species studied and have compared the results with Au Mossbauer data. Trends in isomer shift and quadrupole splitting as a function of ligand are correctly reproduced but the differences between Au(I) and Au(III) compounds are not, due to inadequate flexibility in the basis set, and a strong dependence of Au electron density on total ion charge obscures some of the trends. For a Au(SH) species complexed to SbS(SH)(2)(-1) the Au Mossbauer calculated is in agreement with experiment. The SbS(SH)(2)(-1) unit is produced when OH- attacks a bridging S in solid Sb2S3. Au(SH) can also exothermically attach directly to the bridging S in Sb2S5H4, giving a species with a somewhat different Au Mossbauer isomer shift and quadrupole splitting.