UV Photoelectron Spectra of Some Bent Bis(r;5-cyclopentadienyl)Niobium and Tantalum Complexes

He(I) and He(II) photoelectron spectra are reported for (/^-CsHs^MLX (M = Nb, Ta; L = H, CO; X — propene, 1-butene, propyl) and some methylcyclopentadienyl derivatives. Also the spectra of some yr-allyl and dihalide complexes are reported. Unambiguous assignments could be made of the metal d ai orbital, the Cp orbitals, the olefin n ionization and the M-hydride and 7t-allyl orbitals. Very regular trends are observed in the ionization energies upon substitution of the ligands. The olefin n ionization is found at very low energy indicating that the n 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.