Covalent gold.

Gold is known to be the noblest of all metals because of the relativistic stabilization of its outer 6s orbital. The relativistic effects also lead to destabilization of the 5d orbitals, reducing the 6s-5d energy gap and enhancing s-d hybridization. Therefore, in contrast to its lighter congeners, gold exhibits significant covalent bonding characters and a remarkable repertoire of chemistry, which are increasingly being exploited in catalysis and nanotechnology. This Perspective presents a brief account of recent experimental efforts in the author's laboratory using photoelectron spectroscopy that lead to direct observations of covalent bonding in several relatively simple Au compounds: Au oxides (AuO(-) and AuO(2)(-)), sulfides (AuS(-) and AuS(2)(-)), and the well-known Au(CN)(2)(-) complex. In a series of Au-Si and Au-B mixed clusters, it has also been found that gold atoms behave like H atoms, forming auro-silicon and auro-boron clusters with strong covalent bonding, analogous to the corresponding silicon and boron hydrides, such as the tetrahedral auro-silane (SiAu(4)) versus silane (SiH(4)).