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?

The structural model of the trimethyldioxorhenium molecule, Me3ReO2 (1), has been revised on the basis of analysis of its H-1, C-13, and O-17 NMR and vibrational spectra and of its gas electron diffraction (GED) pattern. The results are consistent with the molecular symmetry C-s; 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) angle CeqReO = 118.0/118.5(10)degrees and angle CeqReCax = 74.3/73.5(11)degrees. The pseudoaxial Re-C bond distance is found to be shorter than the equatorial one, viz., Re-C-ax = 2.130/2.122(6) Angstrom versus Re-C-eq = 2.193/ 2.199(22) Angstrom, and Re=O-av = 1.739/1.703(3) Angstrom. 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 d(z)2 and d(yz) orbitals on the Re atom. The DFT calculations indicate that the axial methyl groups are tilted in such a manner that the angle ReCH valence angles in the ReC3 plane are reduced to 100.8 degrees. It is suggested that this tilting is due in part to bent Re-C-ax bonds and in part to weak C-H ... Re agostic interactions.