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 cyclopentadienyl π character that donate to empty metal d orbitals. A much larger shift of these ionizations is observed upon cyclopentadienyl substitution. The molecular structures are sensitive to the electron configurations. Both density functional theory and ab initio calculations reproduce well the geometry of the neutral molecules and also predict the geometry changes upon ionization. The first ionization, which relates to an orbital with the a1 symmetry of the metal dz2 orbital, is broad due to a substantial geometry change upon removal of an electron from this orbital. The shoulder on the cyclopentadienyl-based ionizations relates to a dynamic Jahn-Teller geometrical distortion. The unusually large metal-to-carbonyl back-bonding observed in these molecules is facilitated by the interligand overlap between the four carbonyls, which substantially stabilizes the appropriate symmetry-adapted carbonyl π* acceptor orbitals. The extensive carbonyl character in the valence electronic structure diminishes any trends in properties with substitutions of the metals down the group or with substitutions on the cyclopentadienyl ring.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry