Fluoronium

The fluoronium ion is an inorganic cation with the chemical formula H
₂F⁺
. It is one of the cations found in fluoroantimonic acid. The structure of the salt with the Sb
₂F⁻
₁₁ anion, has been determined. The fluoronium ion is isoelectronic with the water molecule and the azanide ion.

The term can also refer to organyl substituted species of type H–+F–R, R–+F–R, or R₂C=F⁺. In contrast to the heavier halogens, which have long been known to form open-chain halonium ions (such as [Me₂Cl]⁺[Al(OTeF₅)₄]^–) as well as cyclic haliranium ions, fluorine was not believed to form fluoronium ions of type R–+F–R until the recent characterization of a fluoronium ion locked in a designed cage structure by Lectka and coworkers. Recent solvolysis experiments and NMR spectroscopic studies on a metastable [C–F–C]⁺ fluoronium ion strongly support the dicoordinated fluoronium structure over the alternative rapidly equilibrating classical carbocation. Definitive structural proof of the symmetrical [C–F–C]⁺ was reported by Riedel, Lectka, and coworkers by single crystal X-ray diffraction analysis. Besides its synthesis and crystallographic characterization as the [Sb₂F₁₁]⁻ salt, vibrational spectra could be recorded and a detailed analysis concerning the nature of the bonding situation in this fluoronium ion and its heavier halonium homologues was reported.