The tetrafluoroammonium cation (also known as perfluoroammonium) is a positively charged polyatomic ion with chemical formula . It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.[1] Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane, trifluoramine oxide, tetrafluoroborate anion and the tetrafluoroberyllate anion.
The tetrafluoroammonium ion forms salts with a large variety of fluorine-bearing anions. These include the bifluoride anion, tetrafluorobromate, metal pentafluorides (where M is Ge, Sn, or Ti), hexafluorides (where M is P, As, Sb, Bi, or Pt), heptafluorides (where M is W, U, or Xe), octafluorides,[2] various oxyfluorides (where M is W or U;,), and perchlorate .[3] Attempts to make the nitrate salt,, were unsuccessful because of quick fluorination: + → + .[4]
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:[5]
+ + →
salts can also be prepared by fluorination of with krypton difluoride and fluorides of the form, where M is Sb, Nb, Pt, Ti, or B. For example, reaction of with and yields .[6]
Many tetrafluoroammonium salts can be prepared with metathesis reactions.
Tetrafluoroammonium salts are extremely hygroscopic. The ion, when dissolved in water, readily decomposes into , , and oxygen gas. Some hydrogen peroxide is also formed during this process:
+ → + +
+ 2 → + +
Reaction of with alkali metal nitrates yields fluorine nitrate, .
Because 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.[7]
Tetrafluoroammonium salts usually have no colour. However, some are coloured due to other elements in them., and have a red colour, while,, and are yellow.[7]
salts are important for solid propellant gas generators. They are also used as reagents for electrophilic fluorination of aromatic compounds in organic chemistry.[8] As fluorinating agents, they are also strong enough to react with methane.[9]