[(μ₃ -F)(BrF₅ )₃ ]^- - An Unprecedented Molecular Fluoridobromate(V) Anion in Cs[Br₃ F₁₆ ]

The reaction of Cs[BrF₆ ] with BrF₅ gave the compound Cs[Br₃ F₁₆ ] with the unprecedented propeller-shaped, C₃ -symmetric [(μ₃ -F)(BrF₅ )₃ ]^- anion. All other currently known fluoridobromates(V) contain only octahedral [BrF₆ ]^- anions, which, unlike the related [IF₆ ]^- anions, never exhibited stereochemical activity of the lone pair on the Br atoms. Despite the same coordination number of six for the Br atom in the [BrF₆ ]^- and [(μ₃ -F)(BrF₅ )₃ ]^- anions, the longer μ₃ -F-Br bonds provide additional space, allowing the lone pairs on the Br atoms to become stereochemically active. Cs[Br₃ F₁₆ ] was characterized by single-crystal X-ray diffraction, Raman spectroscopy, and quantum-chemical calculations for both the solid-state compound and the isolated anion at 0 K. Intrinsic bond orbital calculations show that the μ₃ -F-Br bond is essentially ionic in nature and also underpin the stereochemical activity of the lone pairs of the Br(V) atoms.