Topics Covered J. Am. Chem. Soc Electron localization functions show the increasing probability of finding bonding electron pairs between metals and CN– ligands; Cu(CN) 2 – is mostly ionic, whereas Au(CN) 2 – is mostly covalent.

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Experimental evidence has confirmed earlier computational studies showing that the Au–C bonds in Au(CN) 2 – have significant covalent character, in contrast to the ionic behavior of gold's lighter homologs copper and silver. The research, led by Lai-Sheng Wang of Brown University in collaboration with Jun Li of China's Tsinghua University, provides new insight into bonding interactions involving heavier metals and could lead to a better understanding of the recently discovered catalytic properties of gold (J. Am. Chem. Soc. 2009, 131, 16368).

The chemists used gas-phase photoelectron spectroscopy along with theoretical calculations to probe the electronic structure and determine the electron density in M(CN) 2 – for M = Cu, Ag, or Au. They found that the molecular orbitals of the complexes are similar but that the electronic structures are strikingly different, ranging from copper with mostly ionic character to gold with multiple-bond covalent character.

The results indicate that hybridization of s and d orbitals and an increase in electron affinity drive gold to form covalent bonds. Those properties lead to the high stability of Au(CN) 2 –, which has been used since ancient times to extract and process gold.

"Most of the world's gold production and electrolytic gilding is done using gold cyanide," says computational chemist Pekka Pyykö of the University of Helsinki, who has studied multiple-bond character in Au–C bonds. "But it is only now that we have the first thorough study of the complex in the pure gaseous state. It’s a nice piece of work."