Structure of Trimethyldioxorhenium, (CH3)3ReO2, As Studied by Spectroscopic Methods, Gas Electron Diffraction, and Density Functional Theory Calculations. Tilted Methyl Groups: Agostic C−H···M Interactions or Bent M−C Bonds?
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Abstract
The structural model of the trimethyldioxorhenium molecule, Me3ReO2 (1), has been revised on the basis of analysis of its 1H, 13C, and 17O NMR and vibrational spectra and of its gas electron diffraction (GED) pattern. The results are consistent with the molecular symmetry Cs; in the new model both oxo ligands together with one methyl group are located in the equatorial plane of a distorted trigonal bipyramid. Structure optimization by density functional theory (DFT) calculations and least-squares refinement to the GED data yield the valence angles (calc/expt; eq = equatorial; ax = axial; av = average) ∠CeqReO = 118.0/118.5(10)° and ∠CeqReCax = 74.3/73.5(11)°. The pseudoaxial Re−C bond distance is found to be shorter than the equatorial one, viz., Re−Cax = 2.130/2.122(6) Å versus Re−Ceq = 2.193/2.199(22) Å, and ReOav = 1.739/1.703(3) Å. It is suggested that the distortion from trigonal bipyramidal to edge-bridged tetrahedral coordination geometry is driven by the need for the axial C atoms to achieve optimal overlap with both the dz2 and dyz orbitals on the Re atom. The DFT calculations indicate that the axial methyl groups are tilted in such a manner that the ∠ReCH valence angles in the ReC3 plane are reduced to 100.8°. It is suggested that this tilting is due in part to bent Re−Cax bonds and in part to weak C−H···Re agostic interactions.
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