Cs[Cl F ]: A Propeller-Shaped [Cl F ] Anion in a Peculiar A B Structure Type.

Reaction of CsF with ClF leads to Cs[Cl F ]. It contains a molecular, propeller-shaped [Cl F ] anion with a central μ -F atom and three T-shaped ClF molecules coordinated to it. This anion represents the first example of a heteropolyhalide anion of higher ClF content than [ClF ] and is the first Cl-containing interhalogen species with a μ-bridging F atom. The chemical bonds to the central μ -F atom are highly ionic and quite weak as the bond lengths within the coordinating XF units (X = Cl, and also calculated for Br, I) are almost unchanged in comparison to free XF molecules. Cs[Cl F ] crystallizes in a very rarely observed A B structure type, where cations and anions are each pseudohexagonally close packed, and reside, each with coordination number five, in the trigonal bipyramidal voids of the other.