MOLECULAR-STRUCTURE OF BERYLLOCENE, (C5H5)2BE - REINVESTIGATION BY GAS-PHASE ELECTRON-DIFFRACTION

The electron scattering pattern of gaseous dicyclopentadienylberyllium, Cp2Be, has been recorded from s = 2.00 to 39.00 Å-1 with a nozzle temperature of about 120°C. Molecular models of D5d symmetry or models containing one π-bonded and one σ-bonded Cp ring are not compatible with the data. The possibility the gaseous Cp2Be consists fo a mixture D5d and π-Cp, σ-Cp conformers is considered and rejected. A model of C5v symmetry can be brought into satisfactory agreement with the data. It is also found that a slip sandwich model obtained from the C5v model by moving sideways the ring which is at the greatest distance from Be, while keeping the two rings essentially parallel is compatible with the electron diffraction data. The best fit between experimental and calculated intensity curves is obtained with a model with a sideways slip of 0.8(1) Å. This model is similar to that indicated by the X-ray diffraction investigations by Wong and coworkers [4,5]. It is suggested that the potential energy of the molecule does not change much as the magnitude of the slip changes and that the molecule thus undergoes large amplitude vibration. © 1979.