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Tetrafluoroammonium

Chemical compound From Wikipedia, the free encyclopedia

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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.[1] Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane CF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

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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]

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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
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.

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Reactions

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

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]

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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]

See also

References

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