Electronic structure of coordinatively unsaturated molybdenum and molybdenum oxide carbonyls

Results of density functional theory calculations on coordinatively unsaturated molybdenum carbonyl and molybdenum oxide carbonyl anion and neutral complexes observed in previous experimental studies [Wyrwas, Robertson, and Jarrold, J. Chem. Phys. 126, 214309 (2007)] and extended to related complexes are reported. The ground and low-lying electronic states were calculated for the most stable structures predicted for Mo(CO)(n)/Mo(CO)(n) (-) (n=1-3, 5 and 6), MoO(CO)(n)/MoO(CO)(n) (-) (n=0-3), and MoO(2)(CO)(n)/MoO(2)(CO)(n) (-) (n=0-2). Interesting trends are predicted with CO addition, electron addition, and oxidation of the Mo center. In all cases, anions have stronger Mo-CO bond energies, which is attributed to enhanced pi(CO) ( *) backdonation. This enhancement is more dramatic for the molybdenum oxo complexes because the highest occupied molecular orbitals shift from Mo to the pi(CO) ( *) backbonds with the addition of oxygen to the Mo center. Sequential addition of CO for all species results in a sequential stabilization of low spin states and a destabilization of higher spin states. Further, average Mo-CO bond lengths increase as carbonyls are sequentially added. This effect is attributed to fewer electrons per Mo-CO pi(CO) ( *) backbond. Finally, addition of O to Mo(CO)(n) appears to weaken the Mo-CO bonds, and addition of CO to MoO(n) appears to weaken Mo-O bonds. The calculations are validated by favorable agreement between the available measured anion photoelectron spectra and simulated spectra based only on calculated spectroscopic parameters (vibrational frequencies and normal coordinate displacements).