The Electronic Nature of the Metal-Metal Quadruple Bond: Variable Photon Energy Photoelectron Spectroscopy of Mo₂(O₂CCH₃)₄

Variable-energy photoelectron spectroscopy of thin film Mo₂(O₂CCH₃)₄ on a GaAs substrate in ultrahigh vacuum is used to examine the valence ionizations of the quadruple metal-metal bond. The changes in photoionization cross sections with photon energy over a range of 40 to 90 eV are examined. The metal-based σ, π, and δ ionizations of the quadruple bond are strongly enhanced relative to the acetate-based ionizations in the region of photon energies from 40 to 50 eV. This is consistent with a molybdenum 4p to 4d resonance and super-Coster-Kronig Auger enhancement of the metal-based ionizations. The extent of resonance enhancement is related to the amount of Mo 4d character associated with the ionization. The π ionization of the Mo-Mo quadruple bond has the largest contribution from the Mo 4d orbitals. The δ and σ ionizations of the metal-metal bond contain smaller amounts of Mo 4d character, and some Mo 4d character is observed in the acetate-based ionizations. This is explained in terms of significant overlap and mixing of the δ and σ components with the ligand orbitals. Also, in the case of the a component, there is a possible filled/filled interaction between the Mo 4d_z² orbital on one metal and the Mo 4p_z orbital on the adjoining metal. These interactions have significant impact on the properties of these complexes.