EXAFS and Density Functional Study of Gold(I) Thiosulfate Complex in Aqueous Solution

Abstract

The gold thiosulfate complex, Au(S2O3)(2)(3-), has been studied using extended X-ray absorption fine structure (EXAFS) spectroscopy and high level quantum mechanical calculations, which include the effect of aqueous solvent via a dielectric continuum model. EXAFS measurements on Au(S2O3)(2)(3-) show gold coordinated by two sulfurs at 2.29 Angstrom. Density functional calculations, incorporating the effect of solvent using the COSMO method, show the 2-fold S coordination of An to be linear in geometry. This computational approach is further employed to examine the conformational potential energy surface of the complex in aqueous solution. As expected from the significant deviation (108degrees) between the calculated conformation in aqueous solution and the observed crystal structure, internal rotation about the S-Au-S axis is facile: calculation predicts a rotational barrier in solution of less than 3 kcal/mol. Calculated Au-S bond dissociation energies indicate a strong metal-S bond in aqueous solution. Lower solution stability of Au(S2O3)(2)(3-) with respect to another thio gold complex, Au(SH)(2)(-), is predicted in accord with measured overall stability constants. These calculations, together with our experimental observation that Au(S2O3)(2)(3-) decomposes at 393 K, support the view that gold transport by the thiosulfate complex of Au(I) may be restricted to ambient and moderate temperatures. This contrasts with the Au(I) thiolate complex which can mobilize gold at elevated temperature and pressure.