Misconceptions on fluoronium ions and hypervalent fluorine cations

Abstract

In a recent publication (Science,2013,340, 57), indirect experimental evidence was presented for the transient generation in solution of a fluorine atom symmetrically bonded to two carbon atoms. The bridging atom was described as a formally positively charged fluorine engaged in hypervalent bonding, and the molecule was referred to as a fluoronium ion. This interpretation was emphasized in chemistryworld “as the first evidence for hypervalent fluorine cations, or fluoronium ions” (chemistryworld, 4 April,2013), in Chemical & Engineering News as “fluorine’s positive side revealed” (C&E News, 2013, 91, (14) 36), and in a Science perspective as “revealing the positive side of fluorine” (Science, 2013, 340, 41). A critical examination of these interpretations shows that this symmetrically bonded fluorine atom does not carry a positive charge and is not hypervalent or hypercoordinate. Furthermore, it is shown that, in contrast to the “fluoronium ions” which always contain partially negatively charged fluorine atoms, the iodonium cations are true halonium ions according to the IUPAC definition which requires a positive charge on the halogen atom. The calculated partial charges on the halogen atoms in chloronium and bromonium cations are method dependent and can vary from compound to compound, and therefore are more ambiguous.