Ligand effects and periodic trends in metal-metal multiple bonds: Theoretical and experimental studies of electronic structure by gas-phase photoelectron spectroscopy

Abstract

Gas-phase photoelectron spectroscopy was utilized to study a series of metal-metal bonded complexes. Molecular mechanics, Fenske-Hall and ab-initio theoretical calculations were performed on many of the systems. The central focus of all the studies was to identify metal-metal ionizations, and observe how ligands effects influence metal-metal interactions. This information was then used to describe various physical and chemical properties of the species. The first group of compounds studied were molybdenum and tungsten D₃(h) triply-bonded hexa-alkoxides of the form M₂(OC(CH₃)n(CF₃)₃₋n)₆ (where M=Mo, W and n = 1, 2, 3). The metal-metal π and σ ionizations were identified. It is observed that sequential fluorination of the ligands shifts all the valence ionizations. Because the shifts are found to be dominated by charge effects, the virtual levels are assumed to be equally shifted. This information was used to describe the similarity of the UV absorption spectra of the compounds. The next group studied are electron-rich single-bonded Rh(II)-Rh(II) complexes. The previously assigned PES data for Mo₂(O₂CCF₃)₄ was used as a reference point when interpreting the data for Rh₂(O₂CCF₃)₄. The electron configuration orbital ordering for the Rh-Rh bond in Rh₂(O₂CCF₃)₄ is determined to be σ²π⁴δ²δ*²π*⁴, with the δ* and π* being nearly degenerate. It is observed that if the acetates are replaced with more electron-donating ligands, as with Rh₂(O₂CCF₃)₂(form)₂, Rh₂(form)₄ (form = N-N’-p-tolylformamidine, C₁₅H₁₅N₂) and Rh₂(dpf)₄ (dpf = diphenylformamidine, C₁₃H₁₁N₂), the δ* ionizations are destabilized relative to the other Rh-Rh ionizations. In addition, ligand-based ionizations overlap into metal-metal ionizations making it more difficult to interpret data. This information is then used to understand the data and electron structures of Rh₂(pfb)₄ (pfb = perfluorobutyrate, CF₃(CF₂)₂CO₂⁻), Rh₂(capy)₄ (capy = caprolactamate, ⁻OC(CH₂)₅N), Rh₂(OCCH₃NC₆H₅)₄ and Rh₂(OCCH₃NC₆F₅)₄. Rh₂(OCCH₃NC₆H₅)₄ and Rh₂(OCCH₃NC₆F₅)₄ provide examples where charge and overlap effects can be utilized to assign Rh-Rh ionizations. Rh₂(pfb)₄ and Rh₂(capy)₄ exhibit very high and different product selectivities when employed as catalysts in reactions involving carbenes derived from diazo-carbonyl complexes. A mechanism for this catalysis is derived which correlates with the observed selectivities.