Observing the Transition from Equatorial to Axial CO Chemisorption: Infrared Photodissociation Spectroscopy of Yttrium Oxide-Carbonyls.

A series of yttrium oxide–carbonyls are prepared via a laser vaporization supersonic cluster source in the gas phase and identified by mass-selected infrared photodissociation (IRPD) spectroscopy in the C–O stretching region and by comparing the observed IR spectra with those from quantum chemical calculations. For YO(CO)4+, all four CO ligands prefer to occupy the equatorial site of the YO+ unit, leading to a quadrangular pyramid with C4v symmetry. Two energetically nearly degenerate isomers are responsible for YO(CO)5+, in which the fifth CO ligand is either inserted into the equatorial plane of YO(CO)4+ or coordinated opposite the oxygen on the C4 axis. YO(CO)6+ has a pentagonal bipyramidal structure with C5v symmetry, which includes five equatorial CO ligands and one axial CO ligand. The present IRPD spectroscopic and theoretical study of YO(CO)n+ extends the first shell coordination number of CO ligands in metal monoxide carbonyls to six. The transition from equatorial to axial CO chemisorption in th...