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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.[1] Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane CF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.

Tetrafluoroammonium

2D model of the tetrafluoroammonium ion
Names
IUPAC name
Tetrafluoroammonium
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
2028
  • InChI=1S/F4N/c1-5(2,3)4/q+1
    Key: LDOAUKNENSIPAZ-UHFFFAOYSA-N
  • [N+](F)(F)(F)F
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
5
O
where M is W or U; FSO
3
, BrF
4
O
), and perchlorate (ClO
4
).[3] Attempts to make the nitrate salt, NF
4
NO
3
, were unsuccessful because of quick fluorination: NF+
4
+ NO
3
NF
3
+ FONO
2
.[4]

Structure

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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

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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
4
SbF
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
4
AsF
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
4
BF
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
]
2
TiF2−
6
.[7]

Many tetrafluoroammonium salts can be prepared with metathesis reactions.

Reactions

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Tetrafluoroammonium salts are extremely hygroscopic. The NF+
4
ion, when dissolved in water, readily decomposes into NF
3
, H
2
F+
, and oxygen gas. Some hydrogen peroxide (H
2
O
2
) is also formed during this process:[5]

NF+
4
+ H
2
O
NF
3
+ H
2
F+
+ 12 O
2
NF+
4
+ 2 H
2
O
NF
3
+ H
2
F+
+ H
2
O
2

Reaction of NF+
4
SbF
6
with alkali metal nitrates yields fluorine nitrate, FONO
2
.[4]

Properties

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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
)
2
CrF2−
6
, (NF+
4
)
2
NiF2−
6
and (NF+
4
)
2
PtF2−
6
have a red colour, while (NF+
4
)
2
MnF2−
6
, NF+
4
UF
7
, NF+
4
UOF
5
and NF+
4
XeF
7
are yellow.[8]

Applications

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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]

See also

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References

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  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. ^ a b Hoge, B.; Christe, K. O. (2001). "On the stability of NF+
    4
    NO
    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. ^ a b c d e 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. ^ a b 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.