Beryllium cyclopentadienyls

Beryllium forms a series of cyclopentadienyl complexes [Beftj -CsHiY] with Y = H, Cl, Br, Me, —C=CH and BH4, all of which show the expected C5, symmetry (Fig. 5.10a). If the pe/ifo/topfo-cyclopentadienyl group (p. 937) contributes 5 electrons to the bonding, then these are all 8-electron Be complexes consistent with the octet rule for elements of the first short... 

Figure 5.10 Cyclopentadienyl derivatives of beryllium showing (a) the slruclure of fBe(ij -C5H5)Y] and (b) (he structure of crystalline IBe(/j -C5H5)(ij -C5H5)] at —120 (see text).

Cyclopentadienyl derivatives of beryllium are described in reviews that include main-group,35 36 and specifically group 2, metallocenes.4 37... 

An unusual type of organometallic compound of beryllium is that in which it is coordinated to a cyclopentadienyl ring ... 

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 crystal structure of cyclopentadienylberyllium chloride has been determined by X-ray diffraction (280). As portrayed in CIII, the molecule has the same structure in the solid state as in the gas phase. These results rule out the possibility suggested by the mass spectrum (281) that H5C5BeCl is associated. Although not precisely located, the hydrogen atoms of the cyclopentadienyl ring show a slight tendency to bend toward the beryllium atom, in agreement with theoretical predictions (63). 

Among group 2 elements, only beryllium is known to have been inserted into a carborane polyhedron. Even though cyclopentadienyl rr complexes of the heavier alkaline earth meals are known, analogous carborane complexes have not been reported. 

An interesting adaptation of spherical nomenclature occurs when the "central" atom is smaller than one or more of the "ring" atoms. Because carbon is larger than beryllium, consistency requires that the canonical Cartesian name starts with a carbon. In the spherical name, however, this is not a requirement and there is no ambiguity introduced by naming bis(q5-cyclopentadienyl) beryllium as ... 

Fig. 2. The molecular structures of methyl(cyclopentadienyl)beryllium ) and biscyclopentadienyl-beryllium in the gas phase...

In comparison with its beryllium analogue, Cp2Mg has the structure shown in Figure 18.6c, i.e. two r -cyclopentadienyl ligands, and is structurally similar to ferrocene (see Section... 

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