# 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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.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 BeF$^{2−}$ $_4$ 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 (XeF$^{2−}$ $_8$), various oxyfluorides (MF $_5$O$^−$ where M is W or U; FSO$^−$ $_3$, BrF $_4$O$^−$ ), and perchlorate (ClO$^−$ $_4$). Attempts to make the nitrate salt, NF $_4$NO $_3$, were unsuccessful because of quick fluorination: NF$^+$ $_4$ + NO$^−$ $_3$ → NF $_3$ + FONO $_2$.

## Structure

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.

## Synthesis

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:
    NF $_3$ + F $_2$ + SbF $_5$ → NF $_4$SbF $_6$
The hexafluoroarsenate salt was also prepared by a similar reaction with arsenic pentafluoride at 120 °C:
    NF $_3$ + F $_2$ + AsF $_5$ → NF $_4$AsF $_6$
The reaction of nitrogen trifluoride with fluorine and boron trifluoride at 800 °C yields the tetrafluoroborate salt:
    NF $_3$ + F $_2$ + BF $_3$ → NF $_4$BF $_4$
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$^+$ $_4$] $_2$TiF$^{2−}$ $_6$.
Many tetrafluoroammonium salts can be prepared with metathesis reactions.

## Reactions

Tetrafluoroammonium salts are extremely hygroscopic. The NF$^+$ $_4$ ion, when dissolved in water, readily decomposes into NF $_3$, H $_2$F$^+$ , and oxygen gas. Some hydrogen peroxide (H $_2$O $_2$) is also formed during this process:
    NF$^+$ $_4$ + H $_2$O → NF $_3$ + H $_2$F$^+$ + 1⁄2 O $_2$
    NF$^+$ $_4$ + 2 H $_2$O → NF $_3$ + H $_2$F$^+$ + H $_2$O $_2$
Reaction of NF$^+$ $_4$SbF$^−$ $_6$ with alkali metal nitrates yields fluorine nitrate, FONO $_2$.

## Properties

Because NF$^+$ $_4$ salts are destroyed by water, water cannot be used as a solvent. Instead, bromine trifluoride, bromine pentafluoride, iodine pentafluoride, or anhydrous hydrogen fluoride can be used.
Tetrafluoroammonium salts usually have no colour. However, some are coloured due to other elements in them. (NF$^+$ $_4$) $_2$CrF$^{2−}$ $_6$, (NF$^+$ $_4$) $_2$NiF$^{2−}$ $_6$ and (NF$^+$ $_4$) $_2$PtF$^{2−}$ $_6$ have a red colour, while (NF$^+$ $_4$) $_2$MnF$^{2−}$ $_6$, NF$^+$ $_4$UF$^−$ $_7$, NF$^+$ $_4$UOF$^−$ $_5$ and NF$^+$ $_4$XeF$^−$ $_7$ are yellow.

## Applications

NF$^+$ $_4$ salts are important for solid propellant NF $_3$–F $_2$ gas generators. They are also used as reagents for electrophilic fluorination of aromatic compounds in organic chemistry. As fluorinating agents, they are also strong enough to react with methane.