The tetrafluoroammonium

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

(also known as perfluoroammonium) is a positively charged polyatomic ion with chemical formula . It is equivalent to the

ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternar ...

ion where the

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

atoms surrounding the central

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

atom have been replaced by fluorine. Tetrafluoroammonium ion is

isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...

with

tetrafluoromethane Tetrafluoromethane, also known as carbon tetrafluoride or R-14, is the simplest perfluorocarbon ( C F4). As its IUPAC name indicates, tetrafluoromethane is the perfluorinated counterpart to the hydrocarbon methane. It can also be classified as a h ...

trifluoramine oxide Trifluoramine oxide or Nitrogen trifluoride oxide (F3NO) is an inorganic molecule with strong fluorinating powers. Production Trifluoramine oxide was first discovered in 1966 independently by two different groups. One way to produce it was by an e ...

and the

tetrafluoroborate Tetrafluoroborate is the anion . This tetrahedral species is isoelectronic with tetrafluoroberyllate (), tetrafluoromethane (CF4), and tetrafluoroammonium () and is valence isoelectronic with many stable and important species including the perch ...

anion. The tetrafluoroammonium ion forms

salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively ...

with a large variety of fluorine-bearing anions. These include the

bifluoride The bifluoride ion is an inorganic anion with the chemical formula . The anion is colorless. Salts of bifluoride are commonly encountered in the reactions of fluoride salts with hydrofluoric acid. The commercial production of fluorine involves ...

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 (), various oxyfluorides ( where M is W or U; , ), and

perchlorate A perchlorate is a chemical compound containing the perchlorate ion, . The majority of perchlorates are commercially produced salts. They are mainly used as oxidizers for pyrotechnic devices and to control static electricity in food packaging. ...

(). Attempts to make the

nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble ...

salt, , were unsuccessful because of quick fluorination: + → + .

Structure

The geometry of the tetrafluoroammonium ion is

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...

, 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 Nitrogen trifluoride () is an inorganic, colorless, non-flammable, toxic gas with a slightly musty odor. It finds increasing use within the manufacturing of flat-panel displays, photovoltaics, LEDs and other microelectronics. Nitrogen trifluo ...

with fluorine in the presence of a strong

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

which acts as a

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ...

ion acceptor. The original synthesis by Tolberg, Rewick, Stringham, and Hill in 1966 employs antimony pentafluoride as the Lewis acid: : + + → The

hexafluoroarsenate The hexafluoroarsenate (sometimes shortened to fluoroarsenate) anion is a chemical species with formula . Hexafluoroarsenate is relatively inert, being the conjugate base of the notional superacid hexafluoroarsenic acid (). Synthesis The fir ...

salt was also prepared by a similar reaction with

arsenic pentafluoride Arsenic pentafluoride is a chemical compound of arsenic and fluorine. It is a toxic, colorless gas. The oxidation state of arsenic is +5. Synthesis Arsenic pentafluoride can be prepared by direct combination of arsenic and fluorine: :2As + 5F2 ...

at 120 °C: : + + → The reaction of nitrogen trifluoride with fluorine and

boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bondin ...

at 800 °C yields the tetrafluoroborate salt: : + + → salts can also be prepared by fluorination of with

krypton difluoride Krypton difluoride, KrF2 is a chemical compound of krypton and fluorine. It was the first compound of krypton discovered. It is a volatile, colourless solid at room temperature. The structure of the KrF2 molecule is linear, with Kr−F distance ...

() 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 Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

. The ion, when dissolved in water, readily decomposes into , , and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

gas. Some

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...

() is also formed during this process: : + → + + : + 2 → + + Reaction of with alkali metal nitrates yields

fluorine nitrate Fluorine nitrate is an unstable derivative of nitric acid with the formula . It is shock-sensitive. Due to its instability, it is often produced from chlorine nitrate as needed. Synthesis and properties Fluorine nitrate is formed when fluorine ga ...

, .

Properties

Because salts are destroyed by water, water cannot be used as a solvent. Instead,

bromine trifluoride Bromine trifluoride is an interhalogen compound with the formula BrF3. At room temperature, it is a straw-coloured liquid with a pungent odor which decomposes violently on contact with water and organic compounds. It is a powerful fluorinating a ...

bromine pentafluoride Bromine pentafluoride, Br F5, is an interhalogen compound and a fluoride of bromine. It is a strong fluorinating agent. BrF5 finds use in oxygen isotope analysis. Laser ablation of solid silicates in the presence of BrF5 releases O2 for subs ...

iodine pentafluoride Iodine pentafluoride is an interhalogen compound with chemical formula IF5. It is one of the fluorides of iodine. It is a colorless liquid, although impure samples appear yellow. It is used as a fluorination reagent and even a solvent in special ...

, or anhydrous hydrogen fluoride can be used. 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.

Applications

salts are important for solid propellant gas generators. They are also used as reagents for

electrophilic fluorination Electrophilic fluorination is the combination of a carbon-centered nucleophile with an electrophilic source of fluorine to afford organofluorine compounds. Although elemental fluorine and reagents incorporating an oxygen-fluorine bond can be used ...

of aromatic compounds in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

. As fluorinating agents, they are also strong enough to react with methane.

References