Iron(III) fluoride, also known as ferric fluoride, are

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 chemist ...

s with the formula FeF₃(H₂O)_x where x = 0 or 3. They are mainly of interest by researchers, unlike the related

iron(III) chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...

s. Anhydrous iron(III) fluoride is white, whereas the hydrated forms are light pink.Housecroft, Catherine E.; Sharpe, Alan G. (2008) ''Inorganic Chemistry'' (3rd ed.), Pearson: Prentice Hall. .

Chemical and physical properties

Iron(III) fluoride is a thermally robust,

antiferromagnetic In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...

solid consisting of

high spin Spin states when describing transition metal coordination complexes refers to the potential spin configurations of the central metal's d electrons. For several oxidation states, metals can adopt high-spin and low-spin configurations. The ambiguity o ...

Fe(III) centers, which is consistent with the pale colors of all forms of this material. Both anhydrous iron(III) fluoride as well as its hydrates are

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 ...

Structure

The anhydrous form adopts a simple structure with

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

Fe(III)F₆ centres interconnected by linear Fe-F-Fe linkages. In the language of

crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. Crystallography is a fundamental subject in the fields of materials science and solid-state physics (condensed matter physics). The wor ...

, the crystals are classified as rhombohedral with an R-3c

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...

. The structural motif is similar to that seen in ReO₃. Although the solid is nonvolatile, it evaporates at high temperatures, the gas at 987 °C consists of FeF₃, a planar molecule of D_3h

symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...

with three equal Fe-F bonds, each of length 176.3 pm. At very high temperatures, it decomposes to give FeF₂ and F₂. Two crystalline forms—or more technically, polymorphs—of FeF₃·3H₂O are known, the α and β forms. These are prepared by evaporation of an HF solution containing Fe³⁺ at room temperature (α form) and above 50 °C (β form). The

of the β form is P4/m, and the α form maintains a P4/m space group with a J6 substructure. The solid α form is unstable and converts to the β form within days. The two forms are distinguished by their difference in quadrupole splitting from their Mössbauer spectra.

Preparation, occurrence, reactions

Anhydrous iron(III) fluoride is prepared by treating virtually any anhydrous iron compound with fluorine. More practically and like most metal fluorides, it is prepared by treating the corresponding

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...

with hydrogen fluoride: :FeCl₃ + 3 HF → FeF₃ + 3 HCl It also forms as a passivating film upon contact between iron (and steel) and

hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

.J. Aigueperse, P. Mollard, D. Devilliers, M. Chemla, R. Faron, R. Romano, J. P. Cuer, "Fluorine Compounds, Inorganic" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim, 2005. The hydrates crystallize from aqueous hydrofluoric acid. The material is a fluoride acceptor. With

xenon hexafluoride Xenon hexafluoride is a noble gas compound with the formula XeF6. It is one of the three binary fluorides of xenon, the other two being XeF2 and XeF4. All known are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinatin ...

it forms eF₄XeF₅]. Pure FeF₃ is not yet known among minerals. However, hydrated form is known as the very rare

fumarolic mineral Fumarole minerals are minerals which are deposited by fumarole exhalations. They form when gases and compounds Deposition (phase transition), desublimate or precipitate out of condensates, forming mineral deposits. They are mostly associated with v ...

topsøeite. Generally a trihydrate, its chemistry is slightly more complex: FeF _0.5(H₂O)_0.5sub>4·H₂O.

Applications

The primary commercial use of iron(III) fluoride in the production of ceramics."Ferric Fluoride." CAMEO Chemicals. National Oceanic and Atmospheric Administration. Web. 7 Apr. 2010. Some

cross coupling reaction In organic chemistry, a cross-coupling reaction is a reaction where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = organic fragment, M = ...

are catalyzed by ferric fluoride-based compounds. Specifically the coupling of biaryl compounds are catalyzed by hydrated iron(II) fluoride complexes of

N-heterocyclic carbene A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the ''N''-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for ex ...

ligands. Other metal fluorides also

catalyse Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

similar reactions. Iron(III) fluoride has also been shown to catalyze chemoselective addition of cyanide to aldehydes to give the

cyanohydrin In organic chemistry, a cyanohydrin or hydroxynitrile is a functional group found in organic compounds in which a cyano and a hydroxy group are attached to the same carbon atom. The general formula is , where R is H, alkyl, or aryl. Cyanohydrin ...

Safety

The anhydrous material is a powerful dehydrating agent. The formation of ferric fluoride may have been responsible for the explosion of a cylinder of hydrogen fluoride gas.

References

External links

National Pollutant Inventory—Fluoride and compounds fact sheetCAMEO Chemicals: Database of Hazardous Materials
{{DEFAULTSORT:Iron(III) Fluoride Fluorides Metal halides Iron(III) compounds