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A hexafluoride is a chemical compound with the general formula QXnF6, QXnF6m−, or QXnF6m+. Many molecules fit this formula. An important hexafluoride is
hexafluorosilicic acid Hexafluorosilicic acid is an inorganic compound with the chemical formula . Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless. Hexafluo ...
(H2SiF6), which is a byproduct of the mining of
phosphate rock Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxi ...
. In the
nuclear industry Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced ...
,
uranium hexafluoride Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
(UF6) is an important intermediate in the purification of this element.


Hexafluoride cations

Cationic hexafluorides exist but are rarer than neutral or anionic hexafluorides. Examples are the hexafluorochlorine (ClF6+), and hexafluorobromine (BrF6+)
cations 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 con ...
.


Hexafluoride anions

Many elements form anionic hexafluorides. Members of commercial interest are
hexafluorophosphate Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this anio ...
(PF6) and
hexafluorosilicate Hexafluorosilicic acid is an inorganic compound with the chemical formula . Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless. Hexaflu ...
(SiF62−). Many transition metals form hexafluoride anions. Often the monoanions are generated by reduction of the neutral hexafluorides. For example, PtF6 arises by reduction of PtF6 by O2. Because of its highly basic nature and its resistance to oxidation, the fluoride ligand stabilizes some metals in otherwise rare high oxidation states, such as hexafluorocuprate(IV), and
hexafluoronickelate(IV) The fluoronickelates are a class of chemical compounds containing an anion with nickel at its core, surrounded by fluoride ions which act as ligands. This makes it a fluoroanion In chemistry, a fluoroanion or fluorometallate anion is a polyatomi ...
, .


Binary hexafluorides

Seventeen elements are known to form binary hexafluorides. Nine of these elements are
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s, three are
actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
s, four are
chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioac ...
s, and one is a
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
. Most hexafluorides are
molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
compounds with low
melting Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...
and
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
s. Four hexafluorides (S, Se, Te, and W) are gases at room temperature (25 °C) and a pressure of 1 atm, two are liquids (Re, Mo), and the others are volatile solids. The
group 6 Group 6 may refer to: *Group 6 element, chemical element classification *Group 6 (racing) Group 6 was the official designation applied by the FIA to two motor racing classifications, the Prototype-Sports Car category from 1966 to 1971 and the Two ...
,
chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioac ...
, and
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
hexafluorides are colourless, but the other hexafluorides have colours ranging from white, through yellow, orange, red, brown, and grey, to black. The molecular geometry of binary hexafluorides is generally
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 ...
, although some derivatives are distorted from Oh
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 ...
. For the main group hexafluorides, distortion is pronounced for the 14-electron noble gas derivatives. Distortions in gaseous XeF6 are caused by its non-bonding
lone pair In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC ''Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. Lone ...
, according to
VSEPR theory Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm the ...
. In the solid state, it adopts a complex structure involving tetramers and hexamers. According to
quantum chemical Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions ...
calculations, ReF6 and RuF6 should have tetragonally distorted structures (where the two bonds along one axis are longer or shorter than the other four), but this has not been verified experimentally.
Polonium hexafluoride Polonium hexafluoride (PoF6) is a possible chemical compound of polonium and fluorine and one of the seventeen known binary hexafluorides. Synthesis The synthesis of PoF6 via the reaction : + 3 → was attempted in 1945, but the attempt was ...
is known, but not well-studied. It could not be made from 210Po, but work using the longer-lived isotope 208Po and reacting it with fluorine found a volatile product that is almost certainly PoF6. The quoted boiling point in the table below is a prediction.


Binary hexafluorides of the chalcogens


Binary hexafluorides of the noble gases


Binary hexafluorides of the transition metals


Binary hexafluorides of the actinides


Chemical properties of binary hexafluorides

The hexafluorides have a wide range of chemical reactivity.
Sulfur hexafluoride Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non- flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached ...
is nearly inert and non-toxic due to
steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
(the six fluorine atoms are arranged so tightly around the sulfur atom that it is extremely difficult to attack the bonds between the fluorine and sulfur atoms). It has several applications due to its stability, dielectric properties, and high density.
Selenium hexafluoride Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a very toxic colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications. Structure, preparation, and reacti ...
is nearly as unreactive as SF6, but
tellurium hexafluoride Tellurium hexafluoride is the inorganic compound of tellurium and fluorine with the chemical formula TeF6. It is a colorless, highly toxic gas with an unpleasant odor. Preparation Tellurium hexafluoride can be prepared by treating tellurium w ...
is not very stable and can be
hydrolyzed Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...
by water within 1 day. Also, both selenium hexafluoride and tellurium hexafluoride are toxic, while sulfur hexafluoride is non-toxic. In contrast, metal hexafluorides are corrosive, readily hydrolyzed, and may react violently with water. Some of them can be used as
fluorinating agent 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 polyme ...
s. The metal hexafluorides have a high
electron affinity The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy Note that this is ...
, which makes them strong oxidizing agents.
Platinum hexafluoride Platinum hexafluoride is the chemical compound with the formula Pt F6, and is one of seventeen known binary hexafluorides. It is a dark-red volatile solid that forms a red gas. The compound is a unique example of platinum in the +6 oxidation sta ...
in particular is notable for its ability to oxidize the
dioxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others are: *A ...
molecule, O2, to form
dioxygenyl hexafluoroplatinate Dioxygenyl hexafluoroplatinate is a compound with formula O2PtF6. It is a hexafluoroplatinate of the unusual dioxygenyl cation, O2+, and is the first known compound containing this cation. It can be produced by the reaction of dioxygen with platin ...
, and for being the first compound that was observed to react with xenon (see
xenon hexafluoroplatinate Xenon hexafluoroplatinate is the product of the reaction of platinum hexafluoride with xenon, in an experiment that proved the chemical reactivity of the noble gases. This experiment was performed by Neil Bartlett at the University of British Co ...
).


Applications of binary hexafluorides

Some metal hexafluorides find applications due to their volatility.
Uranium hexafluoride Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
is used in the
uranium enrichment Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
process to produce fuel for
nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...
s.
Fluoride volatility Fluoride volatility is the tendency of highly fluorinated molecules to vaporize at comparatively low temperatures. Heptafluorides, hexafluorides and pentafluorides have much lower boiling points than the lower- valence fluorides. Most difluor ...
can also be exploited for
nuclear fuel reprocessing Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the ...
.
Tungsten hexafluoride Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula W F6. It is a toxic, corrosive, colorless gas, with a density of about (roughly 11 times heavier than air). It is one of the densest known gase ...
is used in the production of
semiconductors A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
through the process of
chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...
.


Predicted binary hexafluorides


Radon hexafluoride

Radon hexafluoride (), the heavier homologue of
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 fluorinati ...
, has been studied theoretically, but its synthesis has not yet been confirmed. Higher fluorides of
radon Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through ...
may have been observed in experiments where unknown radon-containing products distilled together 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 fluorinati ...
, and perhaps in the production of radon trioxide: these may have been RnF4, RnF6, or both. It is likely that the difficulty in identifying higher fluorides of radon stems from radon being kinetically hindered from being oxidised beyond the divalent state. This is due to the strong ionicity of RnF2 and the high positive charge on Rn in RnF+. Spatial separation of RnF2 molecules may be necessary to clearly identify higher fluorides of radon, of which RnF4 is expected to be more stable than RnF6 due to spin–orbit splitting of the 6p shell of radon (RnIV would have a closed-shell 6s6p configuration).


Others

Krypton hexafluoride () has been predicted to be stable, but has not been synthesised due to the extreme difficulty of oxidising
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
beyond Kr(II). The synthesis of americium hexafluoride () by the
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, ...
of americium(IV) fluoride () was attempted in 1990, but was unsuccessful; there have also been possible thermochromatographic identifications of it and curium hexafluoride (CmF6), but it is debated if these are conclusive.
Palladium hexafluoride Palladium hexafluoride is an inorganic chemical compound of palladium metal and fluorine with the chemical formula . It is reported to be a still hypothetical compound. This is one of many palladium fluorides. Synthesis Fluorination of palladium ...
(), the lighter homologue of
platinum hexafluoride Platinum hexafluoride is the chemical compound with the formula Pt F6, and is one of seventeen known binary hexafluorides. It is a dark-red volatile solid that forms a red gas. The compound is a unique example of platinum in the +6 oxidation sta ...
, has been calculated to be stable, but has not yet been produced; the possibility of
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
(AgF6) and gold hexafluorides (AuF6) has also been discussed. Chromium hexafluoride (), the lighter homologue of molybdenum hexafluoride and
tungsten hexafluoride Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula W F6. It is a toxic, corrosive, colorless gas, with a density of about (roughly 11 times heavier than air). It is one of the densest known gase ...
, was reported, but has been shown to be a mistaken identification of the known pentafluoride ().


Literature

*


References


Sources

* {{Hexafluorides Hexafluorides