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Tetrafluoroammonium

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Tetrafluoroammonium

2D model of the tetrafluoroammonium ion
Names
IUPAC name Tetrafluoroammonium
Identifiers
CAS Number
3D model (JSmol)
ChEBI
ChemSpider
Gmelin Reference 2028
PubChem CID
CompTox Dashboard (EPA)
InChI
  • InChI=1S/F4N/c1-5(2,3)4/q+1Key: LDOAUKNENSIPAZ-UHFFFAOYSA-N
SMILES
  • (F)(F)(F)F
Properties
Chemical formula F4N
Molar mass 90.000 g·mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references
Chemical compound

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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.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
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
8), various oxyfluorides (MF
5O
where M 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:

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:

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:

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
2TiF
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
2F
, and oxygen gas. Some hydrogen peroxide (H
2O
2) is also formed during this process:

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.

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)
2CrF
6, (NF
4)
2NiF
6 and (NF
4)
2PtF
6 have a red colour, while (NF
4)
2MnF
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. As fluorinating agents, they are also strong enough to react with methane.

See also

References

  1. Nikitin, I. V.; Rosolovskii, V. Y. (1985). "Tetrafluoroammonium Salts". Russian Chemical Reviews. 54 (5): 426. Bibcode:1985RuCRv..54..426N. doi:10.1070/RC1985v054n05ABEH003068. S2CID 250864362.
  2. Christe, K. O.; Wilson, W. W. (1982). "Perfluoroammonium and alkali-metal salts of the heptafluoroxenon(VI) and octafluoroxenon(VI) anions". Inorganic Chemistry. 21 (12): 4113–4117. doi:10.1021/ic00142a001.
  3. Christe, K. O.; Wilson, W. W. (1986). "Synthesis and characterization of tetrafluoroammonium(1+) tetrafluorobromate(1-) and tetrafluoroammonium(1+) tetrafluorooxobromate(1-)". Inorganic Chemistry. 25 (11): 1904–1906. doi:10.1021/ic00231a038.
  4. ^ Hoge, B.; Christe, K. O. (2001). "On the stability of NF
    4NO
    3 and a new synthesis of fluorine nitrate". Journal of Fluorine Chemistry. 110 (2): 87–88. doi:10.1016/S0022-1139(01)00415-8.
  5. ^ Sykes, A. G. (1989). Advances in Inorganic Chemistry. Academic Press. ISBN 0-12-023633-8.
  6. Patnaik, Pradyot (2002). Handbook of inorganic chemicals. McGraw-Hill Professional. ISBN 0-07-049439-8.
  7. John H. Holloway; Eric G. Hope (1998). A. G. Sykes (ed.). Advances in Inorganic Chemistry. Academic Press. pp. 60–61. ISBN 0-12-023646-X.
  8. ^ Sykes, A. G. (1989-07-17). Advances in Inorganic Chemistry. Academic Press. p. 154. ISBN 9780080578828. Retrieved 22 June 2014.
  9. Olah, George A.; Hartz, Nikolai; Rasul, Golam; Wang, Qi; Prakash, G. K. Surya; Casanova, Joseph; Christe, Karl O. (1994-06-01). "Electrophilic Fluorination of Methane with "F+" Equivalent N2F+ and NF4+ Salts". Journal of the American Chemical Society. 116 (13): 5671–5673. doi:10.1021/ja00092a018. ISSN 0002-7863.
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