Electronic structure of early transition-metal carbonyls: Gas-phase photoelectron spectroscopy of (eta(5)-C5H5)M(CO)(4) (M = V, Nb, Ta)

Gas-phase photoelectron spectroscopy is used to investigate the bonding bet ween early transition metals and carbonyl and cyclopentadienyl ligands for the molecules (eta(5)-C5H4R)M- (CO)(4) (R = H, M = V, Nb, Ta; R = SiMe3, M = Nb, Ta; R = COCH3, M = Nb). The lowest ionization energy region contains two overlapping ionizations that arise from the two orbitals that are occup ied according to the formal d(4) metal configuration. However, the characte r of these ionizations is dominated by the carbonyls rather than by the met als, as evidenced by the extensive C-O stretching vibrational progressions observed with these ionizations, by the trends in the ionization cross sect ions between the molecules and with different ionization sources, and by th e relative lack of shifts of these ionizations with metal substitution from vanadium to niobium to tantalum or with trimethylsilyl and acetyl substitu tions on the cyclopentadienyl. The second group of ionizations for these mo lecules corresponds to orbitals with predominantly cyclopentadienyl pi char acter that donate to empty metal d orbitals. A much larger shift of these i onizations is observed upon cyclopentadienyl substitution. The molecular st ructures are sensitive to the electron configurations. Both density functio nal theory and ab initio calculations reproduce well the geometry of the ne utral molecules and also predict the geometry changes upon ionization. The first ionization, which relates to an orbital with the a(1) symmetry of the metal d(z)(2) orbital, is broad due to a substantial geometry change upon removal of an electron from this orbital. The shoulder on the cyclopentadie nyl-based ionizations relates to a dynamic Jahn-Teller geometrical distorti on. The unusually large metal-to-carbonyl back-bonding observed in these mo lecules is facilitated by the interligand overlap between the four carbonyl s, which substantially stabilizes the appropriate symmetry-adapted carbonyl pi* acceptor orbitals. The extensive carbonyl character in the valence ele ctronic structure diminishes any trends in properties with substitutions of the metals down the group or with substitutions on the cyclopentadienyl ri ng.