Nature of the Cycloheptatrienyl-Transition Metal Bond. A Study of Electron Distribution in the Mixed-Ring Sandwich Molecules [Ti(.eta.-C7H7)(.eta.-C5H5)], [Nb(.eta.-C7H7)(.eta.-C5H5)], [Ta(.eta.-C7H7)(.eta.-C5H4Me)], 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.