Study of the Electronic Structure of M-2(CH2CMe3)(6) (M = Mo, W) by Photoelectron Spectroscopy and Density Functional Theory

The valence electronic structures of two dinuclear alkyl compounds containing sigma(2)pi(4) triple bonds between group 6 metals, viz., M-2(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 D-3d (staggered) M2C6 skeleton. For both Mo and W, the highest-energy pair of orbitals are of e(u) (pi) symmetry, followed by one of a(1g) (sigma) symmetry, and comprise the metal-metal triple bond. The orbital energies are higher for W than for Mo, and the separation between the pi and sigma 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 pi ionization of W-2(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 pi and sigma levels vary as a function of the ligand set.