Tetrafluoroammonium

The tetrafluoroammonium cation (also known as perfluoroammonium) is a positively charged polyatomic ion with chemical formula NF⁺
₄. It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine. Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane CF
₄, trifluoramine oxide ONF
₃, tetrafluoroborate BF⁻
₄ anion and the tetrafluoroberyllate BeF²⁻
₄ anion.

The tetrafluoroammonium ion forms salts with a large variety of fluorine-bearing anions. These include the bifluoride anion (HF⁻
₂), tetrafluorobromate (BrF⁻
₄), metal pentafluorides (MF⁻
₅ where M is Ge, Sn, or Ti), hexafluorides (MF⁻
₆ where M is P, As, Sb, Bi, or Pt), heptafluorides (MF⁻
₇ where M is W, U, or Xe), octafluorides (XeF²⁻
₈), various oxyfluorides (MF
₅O⁻
where M is W or U; FSO⁻
₃, BrF
₄O⁻
), and perchlorate (ClO⁻
₄). Attempts to make the nitrate salt, NF
₄NO
₃, were unsuccessful because of quick fluorination: NF⁺
₄ + NO⁻
₃ → NF
₃ + FONO
₂.

The geometry of the tetrafluoroammonium ion is tetrahedral, with an estimated nitrogen-fluorine bond length of 124 pm. All fluorine atoms are in equivalent positions.

Tetrafluoroammonium salts are prepared by oxidising nitrogen trifluoride with fluorine in the presence of a strong Lewis acid which acts as a fluoride ion acceptor. The original synthesis by Tolberg, Rewick, Stringham, and Hill in 1966 employs antimony pentafluoride as the Lewis acid:

The hexafluoroarsenate salt was also prepared by a similar reaction with arsenic pentafluoride at 120 °C:

The reaction of nitrogen trifluoride with fluorine and boron trifluoride at 800 °C yields the tetrafluoroborate salt:

NF⁺
₄ salts can also be prepared by fluorination of NF
₃ with krypton difluoride (KrF
₂) and fluorides of the form MF
_n, where M is Sb, Nb, Pt, Ti, or B. For example, reaction of NF
₃ with KrF
₂ and TiF
₄ yields [NF⁺
₄]
₂TiF²⁻
₆.

Many tetrafluoroammonium salts can be prepared with metathesis reactions.