The electronic structure of several bis(η-arene)metal complexes, synthesized by the metal vapor method, has been examined by electron spin resonance (ESR) and photoelectron spectroscopy (PES). The ESR spectra of Nb(η-arene)2 and Ta(η-C6H6)2 in frozen toluene at 77 K are axial and show g‖ ≈ 2 and g⊥ = 1.94-1.98 consistent with a 2A1 ground state. The spectra show structure arising from coupling to the protons and highly anisotropic coupling to the metal nuclei. The extent of delocalization onto the ring protons is little affected by the arene in the series Nb(η-arene)2 (arene = benzene, toluene, mesitylene), but a substantial increase is observed in the proton coupling down the series V(η-C6H6)2, Nb(η-C6H6)2, and Ta(η-C6H6)2 (AH = 4.1, 5.1, and 6.1 G, respectively). Additionally, solution ESR spectra at 300 K show considerable increases in the metal (n + l)s electron density down the group. The PE spectra were assigned with the aid of comparisons between He I and He II spectra. The spectra of the d6 complexes W(η-arene)2 (arene = benzene, toluene, mesitylene) are consistent with a 2A1 ground state for the cation (IE = 5.2-5.4 eV) with a 2E5/2/2E3/2 doublet appearing ~1.2 eV to higher energy. The first ionizations of the d5 complexes Nb(η-arene)2 are assigned to 1A1 ion states with 3E2 and 1E2 states appearing at higher IE. The ion ground state of Cr(η-C6H5)(η-C6H6) is shown to be the 3E2 state, while the lowest ionization of its molybdenum analogue consists of overlapping bands from the 3E2 and 1A1 ion states. The PE spectra of Ti(η-C6H6)2 and Ti(η-C6H5Me)2 are consistent with a 1A1 ground state leading to a 2E2 ion ground state. Evidence is presented for a back-bonding interaction of e2g symmetry between the metal and the arene ring. © 1983, American Chemical Society. All rights reserved.
