How do coinage metal ions bind to benzene?

The interaction of the metal ions Cu+, Ag+, and Au+ with a benzene molecule has been investigated employing various quantum chemical strategies such as density functionals, Hartree—Fock, second-order perturbation theory and coupled cluster techniques. The three coinage metal cations show interesting differences in their preferred site of complexation with the π system of benzene. Cu+ prefers a highly symmetric η6 arrangement. In the Ag+—C6H6 complex the metal ion can change its position above the whole π plane of benzene virtually barrier free. For Au+ the calculations predict as favoured site of complexation a position above the carbon backbone, i.e., at the periphery and not in the centre of the benzene ring. Unlike copper or silver, this latter site of complexation is significantly less stable in the case of gold. At our most sophisticated level of theory, the computationally predicted binding energies agree well with the experimental numbers for the copper and silver complexes. For the gold complex only an estimate is available experimentally and the calculated number probably provides the most accurate result for this property so far.