Fluoro-nitrogen cations are useful oxidizers, high energy density materials, ingredients in NF₃/F₂ gas generators for chemical lasers, rocket propellants, and electrophilic fluorinating agents. However, little is known about the exact structure and bonding of the simplest members of this class of compounds.

Karl O. Christe, University of Southern California, Los Angeles, USA, and colleagues have determined the crystal structures and bonding of NH₃F⁺, NH₂F₂⁺, and N₂F₃⁺. NH₃F⁺ was prepared by fluorination of carbamates and the difluoroammonium cation through protonation of NHF₂ or FN₃. N₂F₃⁺ antimony salts were obtained from N₂F₄ and SbF₅ in anhydrous HF solution. The cations are challenging to prepare as some compounds are explosive, they are difficult to handle, and their crystal structures are prone to disorder.

The team found that in the series N₂F⁺, N₂F₃⁺, NF₂O⁺, NH₃F⁺, NH₂F₂⁺, and NF₄⁺, the hybridization of the central nitrogen atom and the number of fluorine substituents are the key factors influencing the N-F bond lengths. The N-F bond length decreases from 1.40 Å to 1.26 Å with increasing fluorine substitution and increasing s-characterof the nitrogen atom. High-level quantum chemical calculations were used to support these conclusions. The team claims that the structure–bonding relationships contrast with prior reports because the bonding in the cations is dominated by the s-character of the nitrogen atom and the number of fluorine substituents.

• Fluoro‐nitrogen Cations,
  Karl O. Christe, Ashwani Vij, William W. Wilson, Ralf Haiges, Kyle C. Edwards, David A. Dixon,
  Angew. Chem. Int. Ed. 2021.
  https://doi.org/10.1002/anie.202116565