Beryllocene

The gas-phase decomposition of beryllocenes has been examined for the doping of semiconductor materials and in the preparation of thin metal films. Molecular-beam expitaxy has been used with (MeC5H4)2Be to dope InP semiconductors only a small amount of carbon is found in the doped films.54 Beryllocene 9 has been explored as a precursor for coating capsules with beryllium metal for use as targets in experiments with inertial confinement fusion.55... 

Like the parent Cp2Be, the beryllocenes 10, 11, and 12 are sublimable, air- and oxygen-sensitive solids. They do not react with pyridine, CO, or l,3,4,5-tetramethylimidazol-2-ylidene, but do react with CNC6H3Me2-2,6 to give half-sandwich iminoacyl products via insertion into the Be-C cr-bond, described in Section 2.02.2.3.2. 

Solid-state 9Be and 13C NMR studies have been used to characterize the structure and dynamics of the methyl-substituted beryllocenes. The isotropic 9Be chemical shift has been found to vary with the coordination of the Cp rings, from —19.8ppm in 11 (rf/rf) to — 24.4ppm in 10 (rf/rf). The Cp rings of 11 display comparatively little motion from —100 to 80°C, whereas the rate of reorientation of the CsMes rings decreases substantially at low temperatures in 10.50... 

It was on this background that Haaland et al. reinvestigated the beryllocene stmcture by electron diffraction. They again confirmed the compatibility of the Csv model with the experimental data. However, they also found a slip model by moving the ring at the longest distance from the metal atom sideways to about 0.8 A. The notion of large-amplitude motion is also consistent with the experimental data. 

Further details of the beryllocene stmcture may be expected from more rigorous calculations. However, the stimulating effect of the theoretical calculations on extending the experimental work has already been well demonstrated. 

The synthesis of the first cyclopentadienylberyllium compound was reported by Fischer and Hofmann in 1959 (49). Reaction of cyclopentadienyl alkali-metal compounds with BeCl2 in diethyl ether or benzene leads to dicyclopentadienylberyllium (beryllocene), a rather volatile, colorless, diamagnetic complex, easily soluble in benzene, diethyl ether, and hydrocarbons, and very sensitive to air and moisture. 

In several theoretical papers at different degrees of sophistication attempts have been made to determine the most favorable structure for beryllocene (59-66). Ab initio molecular orbital calculations indicate that in the lowest energy form one pentahapto- and one monohapto-bonded cyclopentadienyl ring are present, as portrayed in structure H. Neither the slip sandwich F nor the off-center double well potential structure G suggested experimentally are indicated to be favorable. Even the symmetrical D5<) sandwich structure... 

Microwave dielectric loss measurements in the region 1-9 GHz have been reported for beryllocene in different solvents (cyclohexane, decalin, benzene, and 1,4-dioxane) (279). The dipolar nature of beryllocene was confirmed. The dielectric absorption was interpreted to suggest rocking movements of the cyclopentadienyl groups synchronous with oscillation of the beryllium atom between two equivalent positions in an rj5, a structure, as indicated in CII. 

The nuances in which details of nomenclature become important occur in those molecules in which not all of the atoms or bonds of a ring are connected to the "spiro" metal atom. For example, returning the focus to Figure 9, Margl, Schwarz and Bloechl [11] advise "Experimental studies find that beryllocene prefers a so-called "slip sandwich" conformation of Cs symmetry in the crystal as well as the gas phase i.e., Be has one t - and one -coordinated ring."... 

Beryllocene, Cp2Be, prepared in 1956 by E. O. Fischer, was intensively examined to determine its structural features. This work quickly abandoned a structural model based on ferrocene, and different bonding modes for the two Cp rings were demonstrated by Cp2Be s high dipole moment in solution. The slipped sandwich structure, with... 

---