Manganese(IV) fluoride
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Manganese tetrafluoride, MnF4, is the highest
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 typ ...
of
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
. It is a powerful
oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...
and is used as a means of purifying elemental
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
..


Preparation

Manganese tetrafluoride was first unequivocally prepared in 1961 by the reaction of
manganese(II) fluoride Manganese(II) fluoride is the chemical compound composed of manganese and fluoride with the formula MnF2. It is a light pink solid, the light pink color being characteristic for manganese(II) compounds. It is made by treating manganese and diverse ...
(or other MnII compounds) with a stream of
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
gas at 550 °C: the MnF4 sublimes into the gas stream and condenses onto a cold finger. This is still the commonest method of preparation, although the sublimation can be avoided by operating at increased fluorine pressure (4.5–6 bar at 180–320 °C) and mechanically agitating the powder to avoid sintering of the grains.. The reaction can also be carried out starting from
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
powder in a fluidized bed. Other preparations of MnF4 include the fluorination of MnF2 with krypton difluoride, or with F2 in liquid
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 ...
solution under
ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
.. Manganese tetrafluoride has also been prepared (but not isolated) in an acid–base reaction between
antimony pentafluoride Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...
and K2MnF6 as part of a chemical synthesis of elemental fluorine.. :K2MnF6 + 2 SbF5 → MnF4 + 2 KSbF6


Chemistry


Decomposition

Manganese tetrafluoride is in equilibrium with manganese(III) fluoride and elemental
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
: :MnF4 MnF3 + F2 Decomposition is favoured by increasing temperature, and disfavoured by the presence of fluorine gas, but the exact parameters of the equilibrium are unclear, with some sources saying that MnF4 will decompose slowly at room temperature, others placing a practical lower temperature limit of 70 °C, and another claiming that MnF4 is essentially stable up to 320 °C.. The equilibrium pressure of fluorine above MnF4 at room temperature has been estimated at about 10−4 Pa (10−9 bar), and the enthalpy change of reaction at .


Other reactions

Manganese tetrafluoride reacts violently with water and even with sodium-dried
petroleum ether Petroleum ether is the petroleum fraction consisting of aliphatic hydrocarbons and boiling in the range 35–60 °C, and commonly used as a laboratory solvent. Despite the name, petroleum ether is not classified as an ether; the term is used ...
. It immediately decomposes on contact with moist air. Reaction with alkali metal fluorides or concentrated hydrofluoric acid gives the yellow hexafluoromanganate(IV) anion nF6sup>2−.


Applications

The main application of manganese tetrafluoride is in the purification of elemental
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
. Fluorine gas is produced by
electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...
of anhydrous
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 ...
(with a small amount of potassium fluoride added as a support electrolyte) in a
Moissan cell Ferdinand Frédéric Henri Moissan (28 September 1852 – 20 February 1907) was a French chemist and pharmacist who won the 1906 Nobel Prize in Chemistry for his work in isolating fluorine from its compounds. Moissan was one of the original mem ...
. The technical product is contaminated with HF, much of which can be removed by passing the gas over solid KF, but also with
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 wel ...
(from traces of water) and possibly heavy-metal fluorides such as arsenic pentafluoride (from contamination of the HF). These contaminants are particularly problematic for the semiconductor industry, which uses high-purity fluorine for etching
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
wafers. Further impurities, such as
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
,
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
,
gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
and
tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...
compounds, can be introduced if unreacted fluorine is recycled. The technical-grade fluorine is purified by reacting it with MnF3 to form manganese tetrafluoride. As this stage, any heavy metals present will form involatile complex fluorides, while the HF and O2 are unreactive. Once the MnF3 has been converted, the excess gas is vented for recycling, carrying the remaining gaseous impurities with it. The MnF4 is then heated to 380 °C to release fluorine at purities of up to 99.95%, reforming MnF3, which can be reused. By placing two reactors in parallel, the purification process can be made continuous, with one reactor taking in technical fluorine while the other delivers high-grade fluorine. Alternatively, the manganese tetrafluoride can be isolated and transported to where the fluorine is needed, at lower cost and greater safety than pressurized fluorine gas.


Fluoromanganate(IV) complexes

The yellow hexafluoromanganate(2−) of
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
and alkaline earth metal cations have been known since 1899, and can be prepared by the fluorination of MnF2 in the presence of the fluoride of the appropriate cation.. They are much more stable than manganese tetrafluoride. Potassium hexafluoromanganate(IV), K2MnF6, can also be prepared by the controlled reduction of potassium permanganate in 50% aqueous
hydrofluoric acid Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...
.. :2 KMnO4 + 2 KF + 10 HF + 3 H2O2 → 2 K2MnF6 + 8 H2O + 3 O2 The pentafluoromanganate(1−) salts of
potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
,
rubidium Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...
and
caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
, MMnF5, can be prepared by fluorination of MMnF3 or by the reaction of nF4(py)(H2O)with MF. The lemon-yellow heptafluoromanganate(3−) salts of the same metals, M3MnF7, have also been prepared.. When potassium hexafluoromanganate is doped into
potassium fluorosilicate Potassium fluorosilicate is a chemical compound with the chemical formula . When doped with Potassium hexafluoromanganate(IV) () it forms a narrow band red producing phosphor, (often abbreviated PSF or KSF), of economic interest due to its applic ...
it forms a narrow band red phosphor.


Notes and references


Notes


References


Further reading

*. *. {{Fluorine compounds Manganese(IV) compounds Fluorides Metal halides Fluorinating agents