Abstract

Recently we established that the ternary complex, CuAsS(SH)(OH) has an unusually high stability and makes a large contribution to the total concentrations of both Cu and As in sulfidic solutions equilibrated with Cu and As sulfide minerals. This ternary complex has an unusual structure, containing a bond which is formally Cu(I)–As(III), along with a broken As–S bond. We have now found that complexes with similar structures exist for Au⁺ and Tl⁺ coordinated to AsS(SH)(OH)⁻. However, such a direct metal–metalloid bond is not a requirement for stability. In fact, TlAsS(SH)(OH) is unstable while AuAsS(SH)(OH) is highly stable (compared to the aquo ion). Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ also form bonds to the As of AsS(SH)(OH), but without breaking any As–S bonds, and HgAsS(SH)(OH)⁺ and PbAsS(SH)(OH)⁺ are particularly stable complexes. Calculated structures are shown for these complexes, gas-phase energies are calculated, and formation constants in aqueous solution are estimated. The SbS(SH)(OH)⁻ ion forms analogous complexes, with similar stabilities. However, the Au⁺ complex of SbS(SH)(OH)⁻ is slightly less stable than the Cu⁺ complex, opposite to the order found for the AsS(SH)(OH)⁻ ligand. The Au⁺ and AuSH complexes of AsSSHOH⁻ or AsS(SH)₂⁻ may be implicated in "invisible gold" in arsenian pyrites. Vibrational frequencies are given for the AuAsS(SH)₃⁻ complex and the XANES energies of this complex and Au(SH)₂⁻ are compared. The existence of such strong complexes may explain the many correlations observed between the concentrations of coinage metals and metalloids.