Tetrafluoroammonium


The tetrafluoroammonium cation 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 It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine. Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane, trifluoramine oxide and the tetrafluoroborate anion.
The tetrafluoroammonium ion forms salts with a large variety of fluorine-bearing anions. These include the bifluoride anion, tetrafluorobromate, metal pentafluorides, hexafluorides, heptafluorides, octafluorides, various oxyfluorides, and perchlorate. Attempts to make the nitrate salt,, were unsuccessful because of quick fluorination: + → +.

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:
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:
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.
Many tetrafluoroammonium salts can be prepared with metathesis reactions.

Reactions

Tetrafluoroammonium salts are extremely hygroscopic. The ion is readily hydrolysed into nitrogen trifluoride, Fluoronium|, and oxygen gas:
Some hydrogen peroxide is also formed during this process.
Reaction of with alkali metal nitrates yields fluorine nitrate,.

Properties

Because tetrafluoroammonium salts are destroyed by water, water cannot be used as a solvent. Instead anhydrous hydrogen fluoride or bromine pentafluoride can be used as a solvent to dissolve these salts.
Tetrafluoroammonium salts usually have no colour. However, some are coloured due to other elements in them. Red salts include, and.,, and are yellow.

Applications

salts are important for solid propellant gas generators. They are also used as reagents for electrophilic fluorination of aromatic compounds in organic chemistry. Its salts are also strong enough fluorinating agents to react with methane.