[FHF]−—The Strongest Hydrogen Bond under the Influence of External Interactions

A search through the Cambridge Structural Database (CSD) for crystal structures containing the [FHF]− anion was carried out. Forty five hydrogen bifluoride structures were found mainly with the H-atom moved from the mid-point of the F…F distance. However several [FHF]− systems characterized by D∞h symmetry were found, the same as this anion possesses in the gas phase. The analysis of CSD results as well as the analysis of results of ab initio calculations on the complexes of [FHF]− with Lewis acid moieties show that the movement of the H-atom from the central position depends on the strength of interaction of this anion with external species. The analysis of the electron charge density distribution in complexes of [FHF]− was performed with the use of the Quantum Theory of Atoms in Molecules (QTAIM) approach and the Natural Bond Orbitals (NBO) method