• Source: 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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.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 CF4, trifluoramine oxide ONF3, 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 (MF5O− where M is W or U; FSO−3, BrF4O−), and perchlorate (ClO−4). Attempts to make the nitrate salt, NF4NO3, were unsuccessful because of quick fluorination: NF+4 + NO−3 → NF3 + FONO2.




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 + F2 + SbF5 → NF4SbF6
The hexafluoroarsenate salt was also prepared by a similar reaction with arsenic pentafluoride at 120 °C:

NF3 + F2 + AsF5 → NF4AsF6
The reaction of nitrogen trifluoride with fluorine and boron trifluoride at 800 °C yields the tetrafluoroborate salt:

NF3 + F2 + BF3 → NF4BF4
NF+4 salts can also be prepared by fluorination of NF3 with krypton difluoride (KrF2) and fluorides of the form MFn, where M is Sb, Nb, Pt, Ti, or B. For example, reaction of NF3 with KrF2 and TiF4 yields [NF+4]2TiF2−6.
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 NF3, H2F+, and oxygen gas. Some hydrogen peroxide (H2O2) is also formed during this process:

NF+4 + H2O → NF3 + H2F+ + 1⁄2 O2
NF+4 + 2 H2O → NF3 + H2F+ + H2O2
Reaction of NF+4SbF−6 with alkali metal nitrates yields fluorine nitrate, FONO2.




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)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.




Applications


NF+4 salts are important for solid propellant NF3–F2 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.




See also


Trifluorooxonium
Nitrogen pentafluoride




References

Kata Kunci Pencarian:

• Tetrafluoroammonium
• Nitrogen pentafluoride
• Nitrogen fluoride
• Ammonium
• Tetrafluoroborate
• Noble gas compound
• Nitrogen trifluoride
• Onium ion
• Trifluorooxonium
• Tetrafluoride