Quadruple metal-metal bonds with strong donor ligands. Ultraviolet photoelectron spectroscopy of M₂(form)₄ (M = Cr, Mo, W; form = N,N'- diphenylformamidinate)

The He I photoelectron spectra of M₂(form)₄ (M = Cr, Mo, W; form = N,N'-diphenylformamidinate) and Mo₂(cyform)₄ (cyform = N,N'- dicyclohexylformamidinate) are presented. For comparison, the Ne I, He I, and He II photoelectron spectra of Mo₂(p-CH₃-form)₄ have also been obtained. The valence ionization features of these molecules are interpreted based on (1) the changes that occur with the metal and ligand substitutions, (2) the changes in photoelectron cross sections with excitation source, and (3) the changes from previously studied dimetal complexes. These photoelectron spectra are useful for revealing the effects that better electron donor ligands have on the valence electronic structure of M₂(L-L)₄ systems. Comparison with the He I spectra of the isoelectronic M₂(O₂CCH₃)₄ compounds is particularly revealing. Unlike with the more electron- withdrawing acetate ligand, several formamidinate-based ionizations derived from the nitrogen pπ orbitals occur among the metal-metal σ, π, and δ ionization bands. Although these formamidinate-based levels are close in energy to the occupied metal-metal bonds, they have little direct mixing interaction with them. The shift of the metal-metal bond ionizations to lower ionization energies for the formamidinate systems is primarily a consequence of the lower electonegativity of the ligand and the better π donation into empty metal levels. The metal-metal δ orbital experiences some additional net bonding interaction with ligand orbitals of the same symmetry. Also, an additional bonding interaction from ligand-to-metal electron donation to the σd* orbital is identified. These spectra suggest a greater degree of metal- ligand covalency than in the related M₂(O₂CCH₃)₄ systems. Fenske-Hall molecular orbital and density functional (ADF) calculations agree with the assignment and interpretation of these spectra. Calculated ionization energies are reported for M₂(form)₄ based on several different density functionals and with different orientations and substitutions for the phenyl rings. It is found that good estimates of the ionization energies are obtained when the truncated system M₂(HN(CH)NH)₄, in which the phenyl groups are replaced by hydrogen atoms, is employed.