Electronic Structure of Early Transition-Metal Carbonyls: Gas-Phase Photoelectron Spectroscopy of (η5-C5H5)M(CO)4 (M = V, Nb, Ta)

Gas-phase photoelectron spectroscopy is used to investigate the bonding between early transition metals and carbonyl and cyclopentadienyl ligands for the molecules (η5-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 occupied according to the formal d4 metal configuration. However, the character of these ionizations is dominated by the carbonyls rather than by the metals, as evidenced by the extensive C−O stretching vibrational progressions observed with these ionizations, by the trends in the ionization cross sections between the molecules and with different ionization sources, and by the relative lack of shifts of these ionizations with metal substitution from vanadium to niobium to tantalum or with trimethylsilyl and acetyl substitutions on the cyclopentadienyl. The second group of ionizations for these molecules corresponds to orbitals with predominantly cycl...