Recently published data for polycarbonyl complexes of monovalent copper, silver, and gold are reviewed and discussed within the context of metal-ligand bonding. For the dicarbonyl complexes [M(CO)(2)](+) the data indicate that (i) metal-carbonyl sigma bonding is strongest for M+ = Au+ and weakest for M+ = Ag+, and (ii) metal-carbonyl pi bonding is strongest for M+ = Cu+ and negligible for M+ = Ag+ and Au+. These variations in M-CO sigma- and pi-bond strengths are explained in terms of the ionization energies and the electronic excited state energies of the three gaseous M+ cations.