STRONG METAL-TO-METAL QUADRUPLE BONDS IN A SERIES OF 5 ISOSTRUCTURAL COMPOUNDS AS INDICATED BY PHOTOELECTRON-SPECTROSCOPY

The He(I) photoelectron spectra of five homologous compounds, MM’(mhp)4, where mhp is the 2-oxo-6-methylpyridine ion and MM’ = Cr2, CrMo, Mo2, MoW, and W2, have been measured; the spectra of the gaseous Hmhp and Hhp (2-hy-droxypyridine) molecules are also reported. The oxidation potentials of the complexes have been recorded by cyclic voltammetry and the Raman spectra of all five compounds are also tabulated (vCr-Mo 504 ± 3 cm-1). On the basis of all of this information, with guidance from MO calculations (not given in detail here), we are able to suggest consistent assignments of the PE spectra leading to the conclusion that the M-M δ bonding electrons have ionization energies of 6.8-5.3 eV (through the series from Cr2 to W2) and that these energies correlate well with the voltammetrically measured E1/2 values. The ionization energies for the molecular orbitals principally responsible for M-M π bonding are fairly constant at about 7.7 eV and the principal σ-bonding orbitals have ionization energies in the range 10.2-10.6 eV. We do not believe that an assignment that places the δ and π ionizations very close together, as suggested by others, is tenable on the basis of all of the evidence available from our experimental studies. © 1979, American Chemical Society. All rights reserved.