The nature of bonding in metal-metal singly bonded coinage metal dimers: Cu₂, Ag₂ and Au₂

The nature of the single bond in the three isoelectronic coinage metal dimers Cu₂, Ag₂ and Au₂ is investigated by means of the ab initio Breathing Orbital Valence Bond (BOVB) method, which allows one to calculate the respective contributions of the covalent and ionic structures to the total wave function, as well as the resonance energy arising from their mixing. It is shown that the BOVB method at its highest level provides bond dissociation energies in very good agreement with reference CCSD(T) values for the three dimers. It is also found that the covalent/ionic resonance energy is important in all three cases, contributing to 40–50% to the total bonding energy, thus qualifying the bonds in Cu₂ and Au₂ as quasi-charge-shift bonds, and that of Ag₂ as a borderline case in-between classical covalent bond and charge-shift one. These results are further confirmed by analyses of the wave functions in terms of the Atoms-in-Molecule theory, which show that the Laplacian of the density at the bond critical point is large and positive in all three cases, which classifies the three bonds as charge-shift bonds within this theory.