Electronic structure investigations of multiple bonding between atoms: From metal-nitrogen triple bonds to metal-metal triple and quadruple bonds

Abstract

The nature of multiple bonding involving transition metal atoms has been explored via photoelectron spectroscopic and computational studies of molecules containing metal-metal quadruple and triple bonds as well as of molecules containing formal metal-nitrogen triple bonds. The principles governing the nature of the multiple bonding in these systems are similar whether the multiple bonding occurs between two transition metals or between a transition metal and a nitrogen atom. First, the electronic structures of the R₃M≡N molecules, where R = ᵗBuO (Cr, Mo, W); iPrO (Mo); (CH₃)₂CF₃CO (Mo); and Cl (Mo), are examined by photoelectron spectroscopy in conjunction with density functional calculations. To assign the features seen in the photoelectron spectra, close attention is paid to the effects of (1) metal substitution and (2) alkoxide (or Cl) substitution. Examination of the photoelectron spectra of the full series of alkoxide-substituted molecules allows the relative positions of the ionizations from the M≡N σ and π orbitals to be identified. Of great importance to the electronic structure of these molecules are the alkoxide orbital combinations that mix strongly with the M≡N σ and π orbitals. The importance of the ancillary ligand combinations is clearly demonstrated by the photoelectron spectroscopic and computational studies of Cl₃Mo≡N. The replacement of the alkoxide ligand with chlorides greatly simplifies the resultant photoelectron spectrum, allowing all of the valence ionizations to be assigned. Next, the bonding in the M₂X₄(PMe₃)₄ molecules, where M = Mo (X = Cl, Br); W (X = Cl); and Re (X = Cl, Br, I), is explored by photoelectron spectroscopic investigations in conjunction with electronic structure calculations. From these investigations, the ionizations from the metal-based orbitals as well as several ligand-based orbitals have been assigned. The first ionization energies of both the molybdenum (δ) and rhenium (δ*) molecules decrease as the electronegativity of the halide increases. The origin of this inverse halide effect is explored. Finally, the nature of the quadruple metal-metal bond in the M₂(chp)₄ molecules (M = Cr, Mo, W; chp = 2-chloro-6-oxo-pyridinate) is probed. For all three metal systems, an ionization from the M₂ δ orbital can be seen. This is only the second time a distinct ionization feature has been noted for ionization of the delta orbital from a dichromium molecule. Comparisons with the previously studied M₂(mhp)₄ molecules (mhp = 6-methyl-2-oxo-pyridinate) allow for a better understanding of the electronic structure of these molecules.