[Br 4 F 21 ] - - a unique molecular tetrahedral interhalogen ion containing a μ 4 -bridging fluorine atom surrounded by BrF 5 molecules.

The reaction of [NMe 4 ][BrF 6 ] with an excess of BrF 5 leads to the compound [NMe 4 ][Br 4 F 21 ]·BrF 5 . It features molecular [(μ 4 -F)(BrF 5 ) 4 ] - anions of tetrahedron-like shape containing central μ 4 -bridging F atoms coordinated by four BrF 5 molecules. It is the most BrF 5 -rich fluoridobromate anion by mass. Quantum-chemical calculations showed that the μ 4 -F-Br bonds within the anion are essentially ionic in nature. The compound is the first example where F atoms bridge μ 4 -like neither to metal nor to hydrogen atoms. It was characterized by Raman spectroscopy and by single-crystal X-ray diffraction. The latter showed surprisingly that its crystal structure is related to the intermetallic half-Heusler compound and structure type MgAgAs.