The term can also refer to organyl substituted species of type H–Шаблон:Overset–R, R–Шаблон:Overset–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]–)[4] as well as cyclic haliranium ions, fluorine was not believed to form fluoronium ions of type R–Шаблон:Overset–R until the recent characterization of a fluoronium ion locked in a designed cage structure by Lectka and coworkers.[5] 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.[6]
