Beryllium organometallics

  Beryllium alkyls and aryls are best made by reaction types 1.1 and 1.2 respectively. They are hydrolysed by water and inflame in air.

                    (1.1)

                 (1.2)

   In the vapour phase, Me₂Be is monomeric, with a linear C^_Be^_C unit (Be^_C = 170pm) . The solid state structure is polymeric (1.1), and resembles that of BeCl₂. However, whereas the bonding in BeCl₂ can be described in terms of a localized bonding scheme , there are insufficient valence electrons available in (Me₂Be)n for an analogous bonding picture. Instead, 3c-2e bonds are invoked as described for BeH₂. Higher alkyls are progressively polymerized to a lesser extent, and the tert-butyl derivative is monomeric under all conditions.

(1.1)

             (1.3)

    Reaction 1.3 leads to the formation of Cp₂Be, and in the solid state, the structure (Figure 1.1a) is in accord with the description (η¹-Cp)(η⁵-Cp)Be. Electron diffraction and spectroscopic studies of Cp₂Be in the gas phase have provided conflicting views of the structure, but recent data indicate that it resembles that found in the solid state rather than the (η⁵-Cp)₂Be originally proposed. In solution, however, the ¹H NMR spectrum shows that all proton environments are equivalent even at 163 K.

Fig. 1.1 (a) The solid state structure of Cp₂Be determined by X-ray diffraction at 128K [K.W. Nugent et al. (1984) Aust. J. Chem., vol. 37, p. 1601]. (b) The same structure showing the two equivalent sites over which the Be atom is disordered. Colour code: Be, yellow; C, grey; H, white.

   Furthermore, the solid state structure is not as simple as Figure 1.1a shows;  The compound (C₅HMe₄)₂Be can be prepared at room temperature from BeCl₂ and K[C₅HMe₄]. In the solid state at 113 K, it is structurally similar to Cp₂Be although, in (C₅HMe₄)₂Be, the Be atom is not disordered. Solution 1H NMR spectroscopic data for (C₅HMe₄)₂Be are consistent with the molecule being fluxional down to 183 K. The fully methylated derivative (C₅Me₅)₂Be is made by reaction 1.4. In contrast to Cp₂Be and (C₅HMe₄)₂Be, (C₅Me₅)₂Be possesses a sandwich structure in which the two C₅-rings are coparallel and staggered (Figure 1.2), i.e. the compound is formulated as (η⁵-C₅Me₅)₂Be.

Fig. 1.2 The solid state structure (X-ray diffraction at 113 K) of (η⁵-C₅Me₅)₂Be [M. del Mar Conejo et al. (2000) Angew. Chem. Int. Ed., vol. 39, p. 1949]. Colour code: Be, yellow; C, grey; H, white.

       (1.4)

   In a sandwich complex, the metal centre lies between two π-bonded hydrocarbon (or derivative) ligands. Complexes of the type (η⁵-Cp)₂M are called metallocenes.