Quadruple metal-metal bonds with strong donor ligands. Ultraviolet photoelectron spectroscopy of M-2(form)(4) (M = Cr, Mo, W; form = N,N '-diphenylformamidinate)

The He I photoelectron spectra of M-2(form)(4) (M = Cr, Mo, W; form = N,N'- diphenylformamidinate) and Mo-2(cyform)(4) (cyform = N,N'-dicyclohexylforma midinate) are presented. For comparison, the Ne I, He I, and He II photoele ctron spectra of Mo-2(p-CH3-form)(4) have also been obtained. The valence i onization features of these molecules are interpreted based on (1) the chan ges that occur with the metal and ligand substitutions, (2) the changes in photoelectron cross sections with excitation source, and (3) the changes fr om previously studied dimetal complexes. These photoelectron spectra are us eful for revealing the effects that better electron donor ligands have on t he valence electronic structure of M-2(L boolean AND L)(4) systems. Compari son with the He I spectra of the isoelectronic M-2(O2CCH3)(4) compounds is particularly revealing. Unlike with the more electron-withdrawing acetate l igand, several formamidinate-based ionizations derived from the nitrogen p( pi) orbitals occur among the metal-metal sigma, pi, and delta ionization ba nds: Although these formamidinate-based levels are close in energy to the o ccupied metal-metal bonds, they have little direct mixing interaction with them. The shift of the metal-metal bond ionizations to lower ionization ene rgies for the formamidinate systems is primarily a consequence of the lower electonegativity of the ligand and the better pi donation into empty metal levels. The metal-metal delta orbital experiences some additional net bond ing interaction with ligand orbitals of the same symmetry. Also, an additio nal bonding interaction from ligand-to-metal electron donation to the delta * orbital is identified. These spectra suggest a greater degree of metal-li gand covalency than in the related M-2(O2CCH3)(4) systems. Fenske-Hall mole cular orbital and density functional (ADF) calculations agree with the assi gnment and interpretation of these spectra. Calculated ionization energies are reported for M-2(form)(4) based on several different density functional s and with different orientations and substitutions for the phenyl rings. I t is found that good estimates of the ionization energies are obtained when the truncated system M-2(HN-(CH)NH)(4), in which the phenyl groups are rep laced by hydrogen atoms, is employed.