Beryllium is a very light metal with high electronegativity, and Be²⁺ is the smallest metal ion. This makes the chemistry of beryllium unique and interesting. However, this chemistry has not been well-studied due to the element’s high toxicity. The halides BeCl₂, BeBr₂, and BeI₂ are commonly used starting materials in the study of beryllium chemistry, but even these compounds’ structures and properties have not been extensively investigated.

For BeBr₂, only a single phase is obtained when the compound is synthesized from these elements. This modification, α-BeBr₂, contains chains of tetrahedrally coordinated Be atoms, μ₂-linked by two Br atoms each. This α-BeBr₂ phase is isostructural to α-BeCl₂. BeCl₂ has a second modification, β-BeCl₂, in which the Be atoms are also tetrahedrally coordinated by the halogen atoms, but these tetrahedra are corner-connected to form supertetrahedra in a 3D network. Analogous α- and β-modifications are known for BeI₂. However, a BeBr₂ phase that is isostructural to β-BeCl₂ and β-BeI₂ had been unknown so far.

Magnus R. Buchner, Carsten von Hänisch, University of Marburg, Germany, and colleagues have synthesized β-BeBr₂ (pictured) via the recrystallization of α-BeBr₂ from benzene in the presence of cyclo-decamethylpentasiloxane as a solubilizer. The product was characterized using single-crystal X-ray diffraction and IR and Raman spectroscopy. The team found that β-BeBr₂ is isostructural to β-BeCl₂ and β-BeI₂, with supertetrahedra forming a three-dimensional coordination network.

• A Second Modification of Beryllium Bromide: β-BeBr₂,
  Magnus R. Buchner, Fabian Dankert, Nils Spang, Florian Pielnhofer, Carsten von Hänisch,
  Inorg. Chem. 2020.
  https://doi.org/10.1021/acs.inorgchem.0c02832