Size and charge effects on the binding of CO to late transition metal clusters

We report on the size and charge dependence of the C-O stretching frequency, ν ( C O ) , in complexes of CO with gas phase anionic, neutral, and cationic cobalt clusters ( Co n C O − ∕ 0 ∕ + ) , anionic, neutral, and cationic rhodium clusters ( Rh n C O − ∕ 0 ∕ + ) , and cationic nickel clusters ( Ni n C O + ) for n up to 37. We develop models, based on the established vibrational spectroscopy of organometallic carbonyl compounds, to understand how cluster size and charge relate to ν ( C O ) in these complexes. The dominating factor is the available electron density for backdonation from the metal to the CO π * orbital. Electrostatic effects play a significant but minor role. For the charged clusters, the size trends are related to the dilution of the charge density at the binding site on the cluster as n increases. At large n , ν ( C O ) approaches asymptotes that are not the same as found for ν ( C O ) on the single crystal metal surfaces, reflecting differences between binding sites on medium sized clusters and the more highly coordinated metal surface sites.