Carbonyl Bonding on Oxophilic Metal Centers: Infrared Photodissociation Spectroscopy of Mononuclear and Dinuclear Titanium Carbonyl Cation Complexes

Mononuclear and dinuclear titanium carbonyl cation complexes including Ti­(CO)₆⁺, Ti­(CO)₇⁺, TiO­(CO)₅⁺, Ti₂(CO)₉⁺ and Ti₂O­(CO)₉⁺ are produced via a laser vaporization supersonic cluster source. The ions are mass selected in a tandem time-of-flight mass spectrometer and studied with infrared photodissociation spectroscopy in the CO stretching frequency region. The structures are established by comparison of the experimental spectra with simulated spectra derived from density functional calculations. Only one IR band is observed for the 15-electron Ti­(CO)₆⁺ cation, which is characterized to have an octahedral <i>O</i>_<i>h</i> structure. The Ti­(CO)₇⁺ cation is determined to be a weakly bound complex involving a Ti­(CO)₆⁺ core ion instead of the seventh coordinated ion. The TiO­(CO)₅⁺ cation has a completed coordination sphere with a C_4v structure. The Ti₂(CO)₉⁺ cation is determined to have a doublet <i>C</i>_<i>s</i> structure with two four-electron donor side-on bridging CO groups and one semibridging CO group. The Ti₂O­(CO)₉⁺ cation has a doublet <i>C</i>_<i>s</i> structure involving a planar cyclic Ti₂O­(η²-μ-CO) core with a four electron donor side-on bridging CO. Bonding analysis indicates that the Ti₂(CO)₉⁺ and Ti₂O­(CO)₉⁺ cations each have a Ti–Ti single bond. The results suggest that metal–metal multiple bonding is not favorable, and the oxophilic titanium centers failed to satisfy the 18-electron configuration in these metal carbonyl complexes