Developing efficient catalysts for methane functionalization is a longstanding goal in inorganic chemistry. Here, we present theoretical calculations to support efforts to synthesize σmethane complexes that can be studied by NMR spectroscopy. The systems studied are osmium complexes of stoichiometry (C 5 R 5 )Os(diphosphine)(CH 3 )(H) + : when both cyclopentadienyl and diphosphine are relatively strong electron donors, the methyl/ hydride structure is in rapid equilibrium with its σ-methane tautomer at low temperatures, as shown experimentally some years ago. Here, using density functional theory, we examine how changing the steric and electronic properties of the ancillary cyclopentadienyl and diphosphine ligands affects the relative energies of the two tautomers, with the goal of identifying a ligand set for which the σ-methane structure, rather than the methyl/hydride form, is the predominant species in equilibrium. We also examine how varying the ancillary ligands affects the barrier for methane dissociation. The calculations suggest that osmium complexes bearing weakly donating and sterically undemanding ligands stabilize the σ-methane structure both relative to its methyl/hydride tautomer and toward dissociation of the methane ligand. More specifically, osmium σ-methane complexes of fluorinated diphosphines (CF 3 ) 2 PCH 2 P(CF 3 ) 2 and (CF 3 ) 2 PCF 2 P(CF 3 ) 2 are predicted to be stable enough to be observed by variable-temperature NMR spectroscopy.

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 D 3d (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.

The activation of inert C─H bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the 1 J CH coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule. These results are relevant to the development of better C─H functionalization catalysts.