[(μ 3 -F)(BrF 5 ) 3 ] - - An Unprecedented Molecular Fluoridobromate(V) Anion in Cs[Br 3 F 16 ].

The reaction of Cs[BrF 6 ] with BrF 5 gave the compound Cs[Br 3 F 16 ] with the unprecedented propeller-shaped, C 3 -symmetric [(μ 3 -F)(BrF 5 ) 3 ] - anion. All other currently known fluoridobromates(V) contain only octahedral [BrF 6 ] - anions, which, unlike the related [IF 6 ] - 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 [BrF 6 ] - and [(μ 3 -F)(BrF 5 ) 3 ] - anions, the longer μ 3 -F-Br bonds provide additional space, allowing the lone pairs on the Br atoms to become stereochemically active. Cs[Br 3 F 16 ] 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 μ 3 -F-Br bond is essentially ionic in nature and also underpin the stereochemical activity of the lone pairs of the Br(V) atoms.