Structural chemistry and geochemistry of silver-sulfur compounds: Critical review

Abstract

Silver(I) bindss srongly with sulfur(II –) in inorganic and organic species, resulting in picomolal aqueous dissolved concentrations. For sulfur species found in the environment, Ag(I)–S(II) bonding forms a linear di-coordinate arrangement, – S–Ag–S–, which results in long zigzag chains in inorganic (minerals) and organic (thiolate) complexes. Silver(I) forms the neutral complex AgHS0 at low concentration of S(II –) and Ag(I). Polynuclear complexes form in solution above micromolal concentrations of S(II –). Silver(I) polysulfides may be significant at elevated S(II –). A number of organic mercaptans (thiols) are found in anoxic sediments in nanomolal to micromolal levels and these can act as ligands for Ag(I). The surprisingly limited, known properties of Ag(I) complexes (thiolates) of environmental mercaptans are reviewed. In these thiolate complexes in the solid phase, single crystal X-ray diffraction shows that the –S(R)–Ag –S(R)– chains condense to form sheets that further link through silver–silver interactions to create a network, or slab of silver and sulfur atoms. From nuclear magnetic resonance spectroscopic evidence, zigzag chains also form in the solution phase and aggregate in a random manner to form colloids, which are the predominant form of Ag(I) in solution. The most crucial and important aspect of Ag(I) thiolate chemistry is the rapid exchange of Ag(I) among thiolates. This process is a mechanism whereby Ag(I) can transfer onto, or off, particulate materials or the cells of an organism. Silver(I) thiolates also react rapidly with H2S or HS− as ligands to form Ag2S, but the reverse process is poor because of the high aqueous insolubility and stability of A2Ss. This reaction represents a possible final fate for any Ag(I) thiolate formed in natural waters, as well as any Ag(I)–S(II –) species.