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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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.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 and the tetrafluoroborate BF−
4 anion.
Contents
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 (XF−
5 where X is Ge, Sn, or Ti), hexafluorides (XF−
6 where X is P, As, Sb, Bi, or Pt), heptafluorides (XF−
7 where X is W, U, or Xe), octafluorides (XeF2−
8), various oxyfluorides (XF
5O−
where X 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.
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:
NF3 + F
2 + SbF
5 → NF
4SbF
6
The hexafluoroarsenate salt was also prepared by a similar reaction with arsenic pentafluoride at 120 °C:
NF3 + F
2 + AsF
5 → NF
4AsF
6
The reaction of nitrogen trifluoride with fluorine and boron trifluoride at 800 °C yields the tetrafluoroborate salt:
NF3 + F
2 + BF
3 → NF
4BF
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]
2TiF2−
6.
Many tetrafluoroammonium salts can be prepared with metathesis reactions.
Reactions
Tetrafluoroammonium salts are extremely hygroscopic. The NF+
4 ion is readily hydrolysed into nitrogen trifluoride, H
2F+
, and oxygen gas:
4 + 2 H
2O → 2 NF
3 + 2 H
2F+
+ O
2
Some hydrogen peroxide (H
2O
2) is also formed during this process.
Reaction of NF+
4SbF−
6 with alkali metal nitrates yields fluorine nitrate, FONO
2.
Properties
Because tetrafluoroammonium salts are destroyed by water, it cannot be used as a solvent. Instead anhydrous hydrogen fluoride or bromine pentafluoride can be used as a solvent to dissolve these salts.
Tetrafluoroammonium salts usually have no colour. However some are coloured due to other metals in them. Red salts include (NF+
4)
2CrF2−
6, (NF+
4)
2NiF2−
6 and (NF+
4)
2PtF2−
6. (NF+
4)
2MnF2−
6, NF+
4UF−
7, NF+
4UOF−
5 and NF+
4XeF−
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.