NATURE OF THE CYCLOHEPTATRIENYL TRANSITION-METAL BOND - A STUDY OF ELECTRON-DISTRIBUTION IN THE MIXED-RING SANDWICH MOLECULES [TI(ETA-C7H(7))(ETA-C5H5)], [NB(ETA-C7H7)(ETA-C5H5)], [TA(ETA-C7H7)(ETA-C(5)H(4)ME)], AND [MO(ETA-C7H7)(ETA-C5H5)] BY PHOTOELECTRON-SPECTROSCOPY WITH VARIABLE PHOTON ENERGY

The photoelectron spectra of [Ti(eta-C7H7)(eta-C5H5)], [Nb(eta-C7H7)(eta C5H5)], [Ta(eta-C7H7)(eta-C(5)H(4)Me)], and [Mo(eta-C7H7)(eta-C5H5)] have been acquired with synchrotron radiation in the incident photon energy range 20-95 eV. Relative partial photoionization cross sections have been derived for the valence bands. The la(1) highest occupied molecular orbital (in [Nb(eta-C7H7)(eta-C5H5)], [Ta(eta-C7H7)(eta-C(5)H(4)Me)], and [Mo(eta-C7H7)(eta-C5H5)]) is confirmed as almost wholly metal-localized. The principal source of metal-cycloheptatrienyl ring covalent bonding is found to be the le(2) molecular orbitals. The mixed metal/ligand character of the le(2) orbitals, estimated as around 60-80% metal character, indicates that neither the +1 nor the -3 formalism for the charge on the cycloheptatrienyl ring is an accurate description of the metal-ring bonding in these complexes. It is considered that three metal valence electrons are required to form the metal-cycloheptatrienyl bond.