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.