[Br₄F₂₁]^- - a unique molecular tetrahedral interhalogen ion containing a μ₄-bridging fluorine atom surrounded by BrF₅...

The reaction of [NMe₄][BrF₆] with an excess of BrF₅ leads to the compound [NMe₄][Br₄F₂₁]·BrF₅. It features molecular [(μ₄-F)(BrF₅)₄]^- anions of tetrahedron-like shape containing central μ₄-bridging F atoms coordinated by four BrF₅ molecules. It is the most BrF₅-rich fluoridobromate anion by mass. Quantum-chemical calculations showed that the μ₄-F-Br bonds within the anion are essentially ionic in nature. The compound is the first example where F atoms bridge μ₄-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.