Evidence of Significant Covalent Bonding in Au(CN)2−

The Au(CN)2− ion is the most stable Au compound known for centuries, yet a detailed understanding of its chemical bonding is still lacking. Here we report direct experimental evidence of significant covalent bonding character in the Au−C bonds in Au(CN)2− using photoelectron spectroscopy and comparisons with its lighter congeners, Ag(CN)2− and Cu(CN)2−. Vibrational progressions in the Au−C stretching mode were observed for all detachment transitions for Au(CN)2−, in contrast to the atomic-like transitions for Cu(CN)2−, revealing the Au−C covalent bonding character. In addition, rich electronic structural information was obtained for Au(CN)2− by employing 118 nm detachment photons. Density functional theory and high-level ab initio calculations were carried out to understand the photoelectron spectra and obtain insight into the nature of the chemical bonding in the M(CN)2− complexes. Significant covalent character in the Au−C bonding due to the strong relativistic effects was revealed in Au(CN)2−, consistent with its high stability.