Is beryllocene a sandwich compound

High-resolution synchrotron powder diffraction data experimental data (dots), calculated for the refined stmcture (line), and allowed Bragg reflections (vertical bars) for Mg(BH4)2-2NH3. The lower trace shows the difference between the experimental data and those calculated for the refined stmcture. Reprinted with permission from [41]. Copyright 2008 American Chemical Society. 

The compounds Be(BH4)(CsH5), Be(BH4)(B5Hio) and Be(B3Hg)2 are all known. What reasonable stmctures could these molecules have, and how could you distinguish between the various possibilities Which, if any, could show fluxional behavior How would you recognize such behavior  

Sandwich compounds such as ferrocene, in which the metal atoms lie between two parallel rings, are described as it-bonded complexes. The cyclopentadienyl groups have local fivefold symmetry and 

After beryllocene was first prepared in 1959, early work showed its IR spectmm to be more complex than those of the symmetrical sandwich molecules such as ferrocene, and it was also found to have a large dipole moment in solution, 2.24 D in cyclohexane. It was thus clear from the start that the molecular symmetry was lower than that of ferrocene, though it was not clear whether this involved two dissimilar rings or two identical but asymmetric rings. An early X-ray diffraction study appeared to show a unit-cell structure formally identical to that of ferrocene, implying a centrosymmetric molecule (12.IVa), which is incompatible with the observation of a dipole moment in solution. Thus there was a possibility that the molecular structure in the crystalline phase was not the same as that in gaseous and solution phases, but it was also possible that the crystallographic results could be attributed to disorder in the crystal. 

The compound is volatile, and the first electron diffraction study suggested a structure that was widely accepted for some years. In this, the two rings were parallel, but the berylhum atom was closer to one than to the other, with distances of 1.48 and 1.98 A to the ring centers, giving overall Csv symmetry, whether the arrangement of the rings was either eclipsed or staggered (12.IVb). An infrared study concluded that the spectra in solid and solution were compatible with this structure. 

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