Silver(I) Fluoride
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Silver(I) fluoride is the
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...
with the formula AgF. It is one of the three main fluorides of silver, the others being silver subfluoride and
silver(II) fluoride Silver(II) fluoride is a chemical compound with the formula AgF2. It is a rare example of a silver(II) compound. Silver usually exists in its +1 oxidation state. It is used as a fluorinating agent. Preparation AgF2 can be synthesized by fluori ...
. AgF has relatively few niche applications; it has been employed as a
fluorination In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...
and desilylation reagent in organic synthesis and in aqueous solution as a topical caries treatment in
dentistry Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions ...
. The hydrates of AgF present as colourless, while pure anhydrous samples are yellow.


Preparation

High-purity silver(I) fluoride can be produced by the heating of silver carbonate to under a hydrogen fluoride environment, in a platinum tube: :Ag2CO3 + 2 HF -> 2 AgF + H2O + CO2 Laboratory routes to the compound typically avoid the use of gaseous hydrogen fluoride. One method is the thermal decomposition of silver tetrafluoroborate: :AgBF4 -> AgF + BF3 In an alternative route, silver(I) oxide is dissolved in concentrated aqueous hydrofluoric acid, and the silver fluoride is precipitated out of the resulting solution by
acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscible wi ...
. :Ag2O + 2 HF -> 2 AgF + H2O


Properties


Structure

The structure of AgF has been determined by X-ray diffraction. At ambient temperature and pressure, silver(I) fluoride exists as the polymorph AgF-I, which adopts a cubic crystal system with space group ''Fmm'' in the Hermann–Mauguin notation. The
rock salt structure In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties o ...
is adopted by the other silver monohalides. The lattice parameter is 4.936(1) Å, significantly lower than those of AgCl and AgBr.
Neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...
and X-ray diffraction studies have further shown that at 2.70(2) GPa, a structural transition occurs to a second polymorph (AgF-II) with the caesium chloride structure, and lattice parameter 2.945 Å. The associated decrease in volume is approximately ten percent. A third polymorph, AgF-III, forms on reducing the pressure to 2.59(2) GPa, and has an inverse nickel arsenide structure. The lattice parameters are a = 3.244(2) Å and c = 6.24(1) Å; the rock salt structure is regained only on reduction of the pressure to 0.9(1) GPa. Non-stochiometric behaviour is exhibited by all three polymorphs under extreme pressures.


Spectroscopy

Silver(I) fluoride exhibits unusual optical properties. Simple electronic band theory predicts that the fundamental exciton absorption for AgF would lie higher than that of AgCl (5.10 eV) and would correspond to a transition from an anionic valence band as for the other silver halides. Experimentally, the fundamental exciton for AgF lies at 4.63 eV. This discrepancy can be explained by positing transition from a valence band with largely silver 4d-orbital character. The high frequency refractive index is 1.73(2).


Photosensitivity

In contrast with the other silver halides, anhydrous silver(I) fluoride is not appreciably photosensitive, although the dihydrate is. With this and the material's solubility in water considered, it is unsurprising that it has found little application in
photography Photography is the visual art, art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It i ...
but may have been one of the salts used by Levi Hill in his " heliochromy", although a US patent for an experimental AgF-based method was granted in 1970.


Solubility

Unlike the other silver halides, AgF is highly soluble in water (1800 g/L), and it even has some solubility in acetonitrile. It is also unique among silver(I) compounds and the silver halides in that it forms the hydrates AgF·(H2O)2 and AgF·(H2O)4 on precipitation from aqueous solution. Like the alkali metal fluorides, it dissolves in hydrogen fluoride to give a conducting solution.


Applications


Organic synthesis

Silver(I) fluoride finds application in organofluorine chemistry for addition of
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 ...
across multiple bonds. For example, AgF adds to perfluoro
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s in acetonitrile to give perfluoroalkylsilver(I) derivatives. It can also be used as a desulfuration-fluorination reagent on thiourea derived substrates. Due to its high solubility in water and organic solvents, it is a convenient source of
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 ...
ions, and can be used to fluorinate alkyl halides under mild conditions. An example is given by the following reaction: : Another organic synthetic method using silver(I) fluoride is the BINAP-AgF complex catalyzed enantioselective protonation of silyl enol ethers: :


Inorganic synthesis

The reaction of silver acetylide with a concentrated solution of silver(I) fluoride results in the formation of a chandelier-like g10sup>2+ cluster with endohedral acetylenediide. Tetralkylammonium fluorides can be conveniently prepared in the laboratory by the reaction of the tetralkylammonium bromide with an aqueous AgF solution.


Other

It is possible to coat a silicon surface with a uniform silver microlayer (0.1 to 1 μm thickness) by passing AgF vapour over it at 60–800 °C. The relevant reaction is: :4 AgF + Si -> 4 Ag + SiF4 Multiple studies have shown silver(I) fluoride to be an effective anti-caries agent, although the mechanism is the subject of current research. Treatment is typically by the "atraumatic" method, in which 40% by mass aqueous silver(I) fluoride solution is applied to carious leisons, followed by sealing of the dentine with glass ionomer cement. Although the treatment is generally recognised to be safe, fluoride toxicity has been a significant clinical concern in paediatric applications, especially as some commercial preparations have had considerable
silver(II) fluoride Silver(II) fluoride is a chemical compound with the formula AgF2. It is a rare example of a silver(II) compound. Silver usually exists in its +1 oxidation state. It is used as a fluorinating agent. Preparation AgF2 can be synthesized by fluori ...
contamination in the past. Due to the instability of concentrated AgF solutions, silver diamine fluoride (Ag(NH3)2F) is now more commonly used. Preparation is by the addition of ammonia to aqueous silver fluoride solution or by the dissolution of silver fluoride in aqueous ammonia.


References

{{DEFAULTSORT:Silver(I) Fluoride Fluorides Silver compounds Metal halides Fluorinating agents Rock salt crystal structure