Infrared photodissociation investigations of metal carbonyl systems in the gas phase
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A brief survey of the history of metal carbonyls and their study is provided, and the methods for generating and studying metal containing species in the gas phase are discussed. In the work presented, gas phase metal ion-ligand complexes are generated with a laser vaporization/supersonic expansion source and studied with infrared photodissociation spectroscopy. Specifically, the atomic metal ion-carbonyl interactions of Au+ and Pt+ with CO and Mg+, Al+ and Ca+ with the organic carbonyl acetone are studied. These complexes' mass spectra and IR photodissociation breakdown patterns give clues about their gas phase coordination number. Metal carbonyl interactions are usually interpreted with the famous Dewar-Chatt-Duncanson (DCD) model. In conjunction with calculations, when available, the IR spectra of these complexes are used to understand the binding interaction and structure. Complexes of Au+(CO)n, n = 3-6 fall under the heading of "nonclassical" metal carbonyls, as evidenced by their vibrational spectra which show shifts of 60-70 cm-1. These complexes have highly symmetric structures as suggested by the single peaks in their IR spectra. Complexes of Pt+(CO)n, n = 4-6, demonstrate only slightly blue shifted spectra of ~ 10 cm-1, and the interaction is suggested to be more of an offsetting synergy in the bonding motifs of the DCD model. In agreement with the previous work of Armentrout and coworkers, our findings indicate that gas phase platinum cation has a preferred coordination number of four that is near square planar. Complexes of M+(acetone), M = Mg, Al and Ca, demonstrate classical red-shifted behavior due to their binding to the carbonyl oxygen. The shift each complex displays relative to free acetone can be interpreted in light of the metal's ionic radius Both the Mg+ and Ca+ complexes have unanticipated doublets in their spectra. These doublets are interpreted as being Fermi resonances with the help of density functional theory. The three studies reported here were aimed at understanding the fundamental interactions of organometallic systems with relevance to inorganic and biochemistry.