UV Photoelectron Spectra of Some Bent Bis(η5-cyclopentadienyl)Niobium and Tantalum Complexes

He(I) and He(II) photoelectron spectra are reported for (η5-C5H5)2MLX (M = Nb, Ta; L = H, CO; X = propene, 1-butene, propyl) and some methylcyclopentadienyl derivatives. Also the spectra of some π-allyl and dihalide complexes are reported. Unambiguous assignments could be made of the metal d a1 orbital, the Cp orbitals, the olefin π ionization and the M-hydride and π-allyl orbitals. Very regular trends are observed in the ionization energies upon substitution of the ligands. The olefin π ionization is found at very low energy indicating that the π backbonding to the olefin is very important. He(I)/He(II) cross section differences show clearly differences in metal character of the various M. O.’s.