Quadruple Metal-Metal Bonds with Strong Donor Ligands. Ultraviolet Photoelectron Spectroscopy of M2(form)4 (M = Cr, Mo, W; form = N,N'-diphenylformamidinate)
详细信息 查看全文
- 作者：Dennis L. Lichtenberger ; Matthew A. Lynn ; and Malcolm H. Chisholm
- 刊名：Journal of the American Chemical Society
- 出版年：1999
- 出版时间：December 29, 1999
- 年：1999
- 卷：121
- 期：51
- 页码：12167 - 12176
- 全文大小：259K
- 年卷期：v.121,no.51(December 29, 1999)
- ISSN：1520-5126
文摘
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, HeI, and He II photoelectron spectra of Mo2(p-CH3-form)4 have also been obtained. The valence ionization featuresof 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 previouslystudied dimetal complexes. These photoelectron spectra are useful for revealing the effects that better electrondonor ligands have on the valence electronic structure of M2(L
f">L)4 systems. Comparison with the He I spectraof 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 p
fchars/pi.gif" BORDER=0 > orbitalsoccur among the metal-metal fchars/sigma.gif" BORDER=0 >, fchars/pi.gif" BORDER=0 >, and fchars/delta.gif" BORDER=0 > ionization bands. Although these formamidinate-based levels areclose in energy to the occupied metal-metal bonds, they have little direct mixing interaction with them. Theshift of the metal-metal bond ionizations to lower ionization energies for the formamidinate systems is primarilya consequence of the lower electonegativity of the ligand and the better fchars/pi.gif" BORDER=0 > donation into empty metal levels.The metal-metal fchars/delta.gif" BORDER=0 > orbital experiences some additional net bonding interaction with ligand orbitals of thesame symmetry. Also, an additional bonding interaction from ligand-to-metal electron donation to the fchars/delta.gif" BORDER=0 >*orbital is identified. These spectra suggest a greater degree of metal-ligand covalency than in the relatedM2(O2CCH3)4 systems. Fenske-Hall molecular orbital and density functional (ADF) calculations agree withthe assignment and interpretation of these spectra. Calculated ionization energies are reported for M2(form)4based on several different density functionals and with different orientations and substitutions for the phenylrings. 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.
f">L)4 systems. Comparison with the He I spectraof 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 pfchars/pi.gif" BORDER=0 > orbitalsoccur among the metal-metal fchars/sigma.gif" BORDER=0 >, fchars/pi.gif" BORDER=0 >, and fchars/delta.gif" BORDER=0 > ionization bands. Although these formamidinate-based levels areclose in energy to the occupied metal-metal bonds, they have little direct mixing interaction with them. Theshift of the metal-metal bond ionizations to lower ionization energies for the formamidinate systems is primarilya consequence of the lower electonegativity of the ligand and the better fchars/pi.gif" BORDER=0 > donation into empty metal levels.The metal-metal fchars/delta.gif" BORDER=0 > orbital experiences some additional net bonding interaction with ligand orbitals of thesame symmetry. Also, an additional bonding interaction from ligand-to-metal electron donation to the fchars/delta.gif" BORDER=0 >*orbital is identified. These spectra suggest a greater degree of metal-ligand covalency than in the relatedM2(O2CCH3)4 systems. Fenske-Hall molecular orbital and density functional (ADF) calculations agree withthe assignment and interpretation of these spectra. Calculated ionization energies are reported for M2(form)4based on several different density functionals and with different orientations and substitutions for the phenylrings. 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.