Significant π bonding in coinage metal complexes OCTMCCO− from infrared photodissociation spectroscopy and theoretical calculations

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A series of coinage metal complexes in the form of TMC(CO)n− (TM = Cu, Ag, Au; n = 0–3) were generated using a laser-ablation supersonic expansion ion source in the gas phase. Mass-selected infrared photodissociation spectroscopy in conjunction with quantum chemical calculations indicated that the TMC(CO)3− complexes contain a linear OCTMCCO− core anion. Bonding analyses suggest that the linear OCTMCCO− anions are better described as the bonding interactions between a singlet ground state TM+ metal cation and the OC/CCO2− ligands in the singlet ground state. In addition to the strong ligands to metal σ donation bonding components, the π-bonding components also contribute significantly to the metal–ligand bonds due to the synergetic effects of the CO and CCO2− ligands. The strengths of the bonding of the three metals show a V-shaped trend in which the second-row transition metal Ag exhibits the weakest interactions whereas the third–row transition metal Au shows the strongest interactions due to relativistic effects.