Study of the Electronic Structure of M2(CH2CMe3)6 (M = Mo, W) by Photoelectron Spectroscopy and Density Functional Theory

The valence electronic structures of two dinuclear alkyl compounds containing σ2π4 triple bonds between group 6 metals, viz., M2(CH2CMe3)6 (M = Mo, W), have been investigated using a combination of molecular orbital theory and variable photon energy photoelectron spectroscopy (PES). Density functional theory (DFT) calculations using PBEO-dDsC functionals, which include dispersion forces, have been performed on the title compounds as well as several closely related M2X6 (M = Mo, W) compounds. The DFT calculations on the dinuclear neopentyl complexes are in excellent agreement with the solid-state structures, measured PES spectra, and ultraviolet–visible (UV–vis) spectra. The top nine filled orbitals in both cases are associated with M–M and M–C bonding. The orbital energy pattern conforms to that anticipated for a D3d (staggered) M2C6 skeleton. For both Mo and W, the highest-energy pair of orbitals are of eu (π) symmetry, followed by one of a1g (σ) symmetry, and comprise the metal–metal triple bond. The orbital energies are higher for W than for Mo, and the separation between the π and σ orbitals is greater for W, reflecting a greater relativistic stabilization of the tungsten 6s orbital compared to that of the Mo 5s orbital. The spin–orbit splitting in the π ionization of W2(CH2CMe3)6 has been resolved and successfully modeled. A graphical comparison of valence orbital energies for Mo2X6, where X = CH2CMe3, NMe2, and OCH2CMe3, shows how the Mo–Mo π and σ levels vary as a function of the ligand set.