Tetrafluoroammonium
Chemical compound
From Wikipedia, the free encyclopedia
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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.[1] Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane CF
4, trifluoramine oxide ONF
3, tetrafluoroborate BF−
4 anion and the tetrafluoroberyllate BeF2−
4 anion.
2D model of the tetrafluoroammonium ion | |
| Names | |
|---|---|
| IUPAC name
Tetrafluoroammonium | |
| Identifiers | |
3D model (JSmol) |
|
| ChEBI | |
| ChemSpider | |
| 2028 | |
PubChem CID |
|
CompTox Dashboard (EPA) |
|
| |
| |
| Properties | |
| F4N+ | |
| Molar mass | 90.000 g·mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
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),[2] various oxyfluorides (MF
5O−
where M is W or U; FSO−
3, BrF
4O−
), and perchlorate (ClO−
4).[3] Attempts to make the nitrate salt, NF
4NO
3, were unsuccessful because of quick fluorination: NF+
4 + NO−
3 → NF
3 + FONO
2.[4]
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.[5]
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:[5]
- NF
3 + F
2 + SbF
5 → NF
4SbF
6
The hexafluoroarsenate salt was also prepared by a similar reaction with arsenic pentafluoride at 120 °C:[5]
- NF
3 + F
2 + AsF
5 → NF
4AsF
6
The reaction of nitrogen trifluoride with fluorine and boron trifluoride at 800 °C yields the tetrafluoroborate salt:[6]
- NF
3 + 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.[7]
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
2F+
, and oxygen gas. Some hydrogen peroxide (H
2O
2) is also formed during this process:[5]
- NF+
4 + H
2O → NF
3 + H
2F+
+ 1⁄2 O
2
- NF+
4 + 2 H
2O → NF
3 + H
2F+
+ H
2O
2
Reaction of NF+
4SbF−
6 with alkali metal nitrates yields fluorine nitrate, FONO
2.[4]
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.[8]
Tetrafluoroammonium salts usually have no colour. However, some are coloured due to other elements in them. (NF+
4)
2CrF2−
6, (NF+
4)
2NiF2−
6 and (NF+
4)
2PtF2−
6 have a red colour, while (NF+
4)
2MnF2−
6, NF+
4UF−
7, NF+
4UOF−
5 and NF+
4XeF−
7 are yellow.[8]
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.[5] As fluorinating agents, they are also strong enough to react with methane.[9]