Hexafluorophosphate is an
anion
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 ...
with chemical formula of . It is an
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 ...
species that imparts no color to its salts. 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 the ...
with
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 ...
, , and the
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 ...
dianion, , and
hexafluoroantimonate
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. ...
. In this anion,
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
has a
valence of 5. Being poorly
nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
, hexafluorophosphate is classified as a
non-coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
.
Synthesis
![Ferrocenium hexafluorophosphate](https://upload.wikimedia.org/wikipedia/commons/7/78/Ferrocenium_hexafluorophosphate.png)
Hexafluorophosphate salts can be prepared by the reaction of
phosphorus pentachloride
Phosphorus pentachloride is the chemical compound with the formula PCl5. It is one of the most important phosphorus chlorides, others being PCl3 and POCl3. PCl5 finds use as a chlorinating reagent. It is a colourless, water-sensitive and moist ...
and alkali or ammonium halide in a solution of
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 ...
:
:
Hexafluorophosphoric acid
Hexafluorophosphoric acid is the inorganic compound with the chemical formula (also written ). This strong Brønsted acid features a non-coordinating anion, hexafluorophosphate (). It is formed from the reaction of hydrogen fluoride with phosp ...
can be prepared by direct reaction of
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
phosphorus pentafluoride
Phosphorus pentafluoride, P F5, is a phosphorus halide. It is a colourless, toxic gas that fumes in air.
Preparation
Phosphorus pentafluoride was first prepared in 1876 by the fluorination of phosphorus pentachloride using arsenic trifluoride, w ...
. It is a strong
Brønsted acid that is typically generated ''
in situ
''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
'' immediately before its use.
:
These reactions require specialized equipment to safely handle the hazards associated with hydrofluoric acid and hydrogen fluoride.
Quantitative analysis
Several methods of
quantitative analysis for the hexafluorophosphate ion have been developed. Tetraphenylarsonium chloride, , has been used both for
titrimetric and
gravimetric
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest.
Units of measurement
G ...
quantifications of hexafluorophosphate. Both of these determinations depend on the formation of tetraphenylarsonium hexafluorophosphate:
:
Hexafluorophosphate can also be determined
spectrophotometrically with
ferroin
Ferroin is the chemical compound with the formula e(''o''-phen)3O4, where ''o''-phen is an abbreviation for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.
Redox ...
.
Reactions
Hydrolysis is extremely slow under basic conditions. Acid-catalyzed
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
to the
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phospho ...
ion is also slow. Nonetheless, hexafluorophosphate is prone to decomposition with the release of hydrogen fluoride in
ionic liquid
An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of ...
s.
Organometallic and inorganic synthesis
Hexafluorophosphate is a common counteranion for
cationic
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 ...
metal complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es. It is one of three widely used non-coordinating anions: hexafluorophosphate,
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 perchlo ...
, 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. Per ...
. Of these, the hexafluorophosphate ion has the least coordinating tendency.
Hexafluorophosphate salts can be prepared by reactions of
silver hexafluorophosphate
Silver hexafluorophosphate, sometimes referred to "silver PF-6," is an inorganic compound with the chemical formula AgPF6.
Uses and reactions
Silver hexafluorophosphate is a commonly encountered reagent in inorganic and organometallic chemistry. ...
with halide salts. Precipitation of insoluble silver halide helps drive this reaction to completion. Since hexafluorophosphate salts are often insoluble in water but soluble in polar organic solvents, even the addition of
ammonium hexafluorophosphate
Ammonium hexafluorophosphate is the inorganic compound with the formula NH4PF6. It is a white water-soluble, hygroscopic solid. The compound is a salt consisting of the ammonium cation and hexafluorophosphate anion. It is commonly used as a source ...
() to aqueous solutions of many organic and inorganic salts gives solid precipitates of hexafluorophosphate salts. One example is the synthesis of
rhodocenium salts: The overall conversion equation is
:
is produced by the addition of hexafluorophosphoric acid to a suspension of
copper(I) oxide
Copper(I) oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper, the other being or copper(II) oxide or cupric oxide (CuO). This red-coloured solid is a component of some antifoulin ...
in acetonitrile:
:
Hydrolysis of hexafluorophosphate complexes
While the hexafluorophosphate ion is generally inert and hence a suitable
counterion
160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion.">cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typical ...
, its
solvolysis
In chemistry, solvolysis is a type of nucleophilic substitution (S1/S2) or elimination reaction, elimination where the nucleophile is a solvent molecule. Characteristic of S1 reactions, solvolysis of a chirality (chemistry), chiral reactant affor ...
can be induced by highly electrophilic metal centers. For example, the tris(
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
o)
rhodium
Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isoto ...
complex undergoes solvolysis when heated in
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 miscib ...
, forming a
difluorophosphate-bridged complex .
:
Applications
Practical uses of the hexafluorophosphate ion typically exploit one or more of the following properties: that it is a
non-coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
; that hexafluorophosphate compounds are typically soluble in organic solvents, particularly
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
* Polar climate, the c ...
ones, but have low solubility in
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
; or, that it has a high degree of stability, including resistance to both acidic and basic
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
.
Secondary batteries
The main commercial use of hexafluorophosphate is as its lithium salt,
lithium hexafluorophosphate
Lithium hexafluorophosphate is an inorganic compound with the formula Li PF6. It is a white crystalline powder.
Production
LiPF6 is manufactured by reacting phosphorus pentachloride with hydrogen fluoride and lithium fluoride
:PCl5 + LiF + ...
. This salt, in combination with
dimethyl carbonate
Dimethyl carbonate (DMC) is an organic compound with the formula OC(OCH3)2. It is a colourless, flammable liquid. It is classified as a carbonate ester. This compound has found use as a methylating agent and more recently as a solvent that is ...
, is a common electrolyte in commercial
secondary batteries
A rechargeable battery, storage battery, or secondary cell (formally a type of energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to a disposable or prim ...
such as
lithium-ion cells. This application exploits the high solubility of hexafluorophosphate salts in organic solvents and the resistance of these salts to reduction by the alkali metal cathode. Since the lithium ions in these batteries are generally present as
coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es within the electrolyte, the non-coordinating nature of the hexafluorophosphate ion is also a useful property for these applications.
Ionic liquids
Room temperature ionic liquids such as
1-butyl-3-methylimidazolium hexafluorophosphate (typically abbreviated as bmimPF
6) have been prepared.
The advantage of the anion exchange in favour of a non-coordinating anion is that the resulting ionic liquid has much greater thermal stability. 1-Butyl-3-methylimidazolium chloride decomposes to
''N''-methylimidazole and
1-chlorobutane or to ''N''-butylimidazole and
chloromethane
Chloromethane, also called methyl chloride, Refrigerant-40, R-40 or HCC 40, is an organic compound with the chemical formula . One of the haloalkanes, it is a colorless, odorless, flammable gas. Methyl chloride is a crucial reagent in industrial ...
. Such decompositions are not possible for bmimPF
6. However, thermal decompositions of hexafluorophosphate ionic liquids to generate
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 ...
gas are known.
:
References
{{Hexafluorophosphates
*
Non-coordinating anions