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Beryllium (atomic symbol: Be, atomic number: 4) is a silver-white metal with a very low density and high thermal conductivity. It is used in high-technology applications because of its light weight and X-ray transparency. However, beryllium is also one of the most toxic metals. Its toxicity arises from the formation of beryllium ions in solution, and it is therefore typically used in solid forms such as beryllium bromide.
beryllium bromide is a white crystalline chemical compound with the formula BeBr2. It is highly hygroscopic and dissolves in water. Two polymorphs are known, a form with edge-sharing tetrahedral Be2+ centres and a form resembling zinc iodide with interconnected adamantane-like cages. Both forms of the molecule are stable at room temperature.
b-BeBr2 crystallizes in the isometric system and has very small bulk moduli, which indicate strong hydrogen bonding within the structure. The b modification of the Be-Br bond is energetically preferred over the a modification. The b-BeBr2 form is 6.6 kJ mol-1 more stable than the a-BeBr2, which is in agreement with the lower energy required for breaking the beryllium-bromide bonds by solvation.
The preparation of b-BeBr2 has been described by several groups. The most convenient method involves the reaction of BeCl2 with siloxanes or partially silicon-based crown ethers. The reaction is rapid and gives a high yield of b-BeBr2. The synthesis can be performed under normal conditions, without the use of a protective atmosphere, because beryllium halides do not react with oxygen or nitrogen containing solvents or ligands. Nevertheless, care must be taken with all reactions of beryllium compounds because of the potential dangers associated with their inhalation and ingestion.