Syntheses and Crystal Structures of Brominated Polyhedral Arsa‐ and Phosphaboranes

The synthesis of the perbrominated arsaboranes closo‐1,2‐As₂B₄Br₄ ( 1 ) and closo‐1,2‐As₂B₁₀Br₁₀ ( 2 ) occurs by co‐pyrolysis of B₂Br₄ and AsBr₃ at 500 °C. Repeated fractionation of the sublimable products in vacuo yields both compounds in pure form. The X‐ray structure determination for orthorhombic closo‐1,2‐As₂B₄Br₄ ( 1 ) space group: Pbcn, a = 2345.48(17) pm, b = 627.31(4) pm, c = 1294.02(9) pm for Z = 8] and the corresponding phosphorus compound, monoclinic closo‐1,2‐P₂B₄Br₄ ( 3 ) space group: P2₁/n, a = 806.84(6) pm, b = 1247.96(9) pm, c = 974.91(7) pm, β = 90.493(3)° and Z = 4] confirmed that both 1 and 3 , consistent with their 14‐skeletal electron counts, adopt octahedral structures distorted from regular by two arsenic or phosphorus atoms in the 1,2‐positions. The shortest boron–boron bonds within the cluster frameworks are located between the boron atoms antipodal to the heteroatoms.