Quadruple Metal−Metal Bonds with Strong Donor Ligands. Ultraviolet Photoelectron Spectroscopy of M₂(form)₄ (M = Cr, Mo, W; form = N,N‘-diphenylformamidinate)

Abstract

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 δ* 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.

Closed-Shell Molecules That Ionize More Readily Than Cesium

“…The He self-ionization, which appears as the sharp band at 4.99 eV on the He I IE scale, as shown in the spectrum of the Mo 2 (hpp) 4 molecule, is a convenient reference point for the low IEs (20). The trend in the IEs from the dichromium to the dimolybdenum to the ditungsten molecule is exactly as expected based on previous studies of quadruple-bonded dimetal molecules with strong donor ligands (21). For example, Ta-ble 2 shows a comparison of the first IEs of these molecules with the first IEs of the related M 2 (DPhF) 4 molecules (DPhF is N 2 NЈ diphenylformamidinate).…”

Physical, Spectroscopic and Theoretical Results

“…A major study dealt with the M 2 (DPhF) 4 molecules (M = Cr, Mo, W) and the Mo 2 (DCyF) 4 molecule. 309 It was found that in contrast to the acetate compounds, several formamidinatebased ionizations derived from the nitrogen p orbitals occur among the metal-metal , , and ionization bands. Although these formamidinate-based levels are close in energy to the occupied metal-metal bonding orbitals, there is little direct mixing.…”

Synthesis and structural characterisation of some novel lithium and sodium N,N′-di(para-tolyl)formamidinate complexes

“…It is interesting to note that the structural motif of the anion is reminiscent of the paddle-wheel structure that has been found for many transition metals using similar ligand systems. 15,16, 78 Considering lithium is smaller than most transition metals it is understandable that only three formamidinate ligands are able to orientate themselves around the two lithium centres, compared with four in the transition metal complexes.…”