Hexafluorophosphate
Encyclopedia
Hexafluorophosphate is an anion with chemical formula of . This octahedral
species is isoelectronic with sulfur hexafluoride
, SF6, and is valence isoelectronic with the highly stable superacid
anion fluoroantimonate
. As a non-coordinating anion
, it is a poor nucleophile
. It is prone to decomposition with the release of hydrogen fluoride
in ionic liquid
s but is generally extremely stable in solution. Hydrolysis
to the phosphate
ion is very slow even in concentrated acid
with warming, and even slower under basic conditions.
:
Hexafluorophosphoric acid
can be prepared by direct reaction of hydrogen fluoride
with phosphorus pentafluoride
. It is a strong Brønsted acid that is typically generated in situ immediately prior to its use.
These reactions require specialized equipment to safely handle the hazards associated with hydrofluoric acid solution and hydrogen fluoride gas.
is an area of analytical chemistry
used to determine the amount of a species present in a sample. Several methods of quantitative analysis for the hexafluorophosphate ion have been developed. Tetraphenylarsonium chloride, [(C6H5)4As]Cl, has been used both for titrimetric
and gravimetric
quantifications of hexafluorophosphate. Both of these determinations depend on the formation of tetraphenylarsonium hexafluorophosphate:
Hexafluorophosphate can also be determined spectrophotometrically
with ferroin
.
s are anions that interact only weakly with cations, a useful property when studying highly electrophilic cations. In coordination chemistry, the term can also be used to refer to anions which are unlikely to bind directly to the metal centre of a complex. Hexafluorophosphate is a non-coordinating anion in both senses of the term. Three widely used non-coordinating anions are hexafluorophosphate, tetrafluoroborate
, and perchlorate
; of these, the hexafluorophosphate ion has the least coordinating ability and it is deliberately used for this property. In the 1990s, a new non-coordinating anion, [B[3,5-(CF3)2C6H3]4]− commonly abbreviated as [BArF4]− and colloquially known as "BARF" was discovered; BARF is far less coordinating than hexafluorophosphate.
; that hexafluorophosphate compounds are typically soluble in organic solvents, particularly polar
ones, but have low solubility in aqueous solution
; or, that it has a high degree of stability, including resistance to both acidic and basic hydrolysis
.
. This salt in combination with dimethylcarbonate, is a common electrolyte in commercial secondary batteries 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 complexes within the electrolyte, the non-coordinating nature of the hexafluorophosphate ion is also a useful property for these applications.
syntheses
to provide an inert and non-coordinating counterion. Tetrafluoroborate
salts are a common alternative choice. One route to such compounds involves reactions with silver hexafluorophosphate
with the halide salt. Precipitation of insoluble silver halide salt 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
(NH4PF6) to aqueous solutions of many organic and inorganic salts gives solid precipitates of hexafluorophosphate salts. This method can have advantages over the silver hexafluorophosphate method in terms of expense and in systems where contamination with metal ions is strongly discouraged. As an example, the microwave synthesis
of rhodicinium hexafluorophosphate
involves the reaction of cyclopentadiene
and rhodium(III) chloride hydrate
in methanol
. During the workup with methanolic ammonium hexafluorophosphate the product salt precipitates cleanly from the reaction mixture. The overall conversion equation is
Whilst the hexafluorophosphate ion is generally considered inert and hence a suitable counterion
, solvolysis
reactions of the hexafluorophosphate ion are known. For example, the tris(solvent
) rhodium
complex [(η5-C5Me5)Rh(Me2CO)3](PF6)2 undergoes solvolysis when heated in acetone
, forming a difluorophosphate-bridged complex [(η5-C5Me5)Rh(μ-OPF2O)3Rh(η5-C5Me5)]PF6.
(typically abbreviated as bmimPF6) have been prepared. The advantage of the anion exchange in favour of a non-coordinating anion is that the resulting ionic liquid has much higher thermally stability. The reason is that the possibility of decomposition of the imidazolium cation is decreased; 1-butyl-3-methylimidazolium chloride could decompose to N-methylimidazole
and 1-chlorobutane
or to N-butylimidazole and chloromethane
. Such decompositions are not possible for bmimPF6 However, thermal decompositions of hexafluorophosphate ionic liquids to generate hydrogen fluoride
gas are known.
, [Cu(CH3CN)4]PF6, the acetonitrile
ligand
s protect the copper(I) centre from oxidation to copper(II); combined with the lability of the complex, this makes for a suitable precursor in the non-aqueous syntheses of copper(I) compounds. However, the nearly linear arrangement of the 'arms' of this tetrahedral cation makes for a high degree of bulk. Consequently, the inclusion of a bulky counterion has a stabilising effect and a non-coordinating anion like hexafluorophosphate is ideal given the lability. The compound can be produced by the addition of hexafluorophosphoric acid to a suspension of copper(I) oxide
in acetonitrile:
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
species is isoelectronic with sulfur hexafluoride
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
, SF6, and is valence isoelectronic with the highly stable superacid
Superacid
According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...
anion fluoroantimonate
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....
. As a non-coordinating anion
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 as counterions for cationic metal complexes...
, it is a poor nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
. It is prone to decomposition with the release of hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
in ionic liquid
Ionic liquid
An ionic liquid is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ILs are largely made...
s but is generally extremely stable in solution. Hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
to the phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
ion is very slow even in concentrated acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
with warming, and even slower under basic conditions.
Synthesis
Hexafluorophosphate salts can be prepared by the reaction of phosphorus pentachloride and alkali or ammonium halide in a solution of hydrofluoric acidHydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
:
- PCl5 + MCl + 6 HF → MPF6 + 6 HCl
Hexafluorophosphoric acid
Hexafluorophosphoric acid
Hexafluorophosphoric acid is the chemical compound with the chemical formula HPF6. This strong Brønsted acid features a non-coordinating anion, hexafluorophosphate . It is formed from the reaction of hydrogen fluoride with phosphorus pentafluoride....
can be prepared by direct reaction of hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
with phosphorus pentafluoride
Phosphorus pentafluoride
Phosphorus pentafluoride, PF5, is a phosphorus halide. It's a colourless gas at room temperature and pressure.-Structure:Single-crystal X-ray studies indicate PF5 molecule has two distinct P−F bonds : P−Fax = 158.0 pm and P−Feq = 152.2 pm...
. It is a strong Brønsted acid that is typically generated in situ immediately prior to its use.
- PF5 + HF → HPF6
These reactions require specialized equipment to safely handle the hazards associated with hydrofluoric acid solution and hydrogen fluoride gas.
Quantitative analysis
Quantitative analysisQuantitative analysis (chemistry)
In chemistry, quantitative analysis is the determination of the absolute or relative abundance of one, several or all particular substance present in a sample....
is an area of analytical chemistry
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
used to determine the amount of a species present in a sample. Several methods of quantitative analysis for the hexafluorophosphate ion have been developed. Tetraphenylarsonium chloride, [(C6H5)4As]Cl, has been used both for titrimetric
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
and gravimetric
Gravimetric analysis
Gravimetric analysis describes a set of methods in analytical chemistry for the quantitative determination of an analyte based on the mass of a solid...
quantifications of hexafluorophosphate. Both of these determinations depend on the formation of tetraphenylarsonium hexafluorophosphate:
- [(C6H5)4As]+ + PF6− → [(C6H5)4As]PF6
Hexafluorophosphate can also be determined spectrophotometrically
Spectrophotometry
In chemistry, spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength...
with ferroin
Ferroin
Ferroin is the chemical compound with the formula [Fe3]SO4, 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 indicator:...
.
Properties
Non-coordinating anionNon-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 as counterions for cationic metal complexes...
s are anions that interact only weakly with cations, a useful property when studying highly electrophilic cations. In coordination chemistry, the term can also be used to refer to anions which are unlikely to bind directly to the metal centre of a complex. Hexafluorophosphate is a non-coordinating anion in both senses of the term. Three widely used non-coordinating anions are hexafluorophosphate, tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...
, and perchlorate
Perchlorate
Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...
; of these, the hexafluorophosphate ion has the least coordinating ability and it is deliberately used for this property. In the 1990s, a new non-coordinating anion, [B[3,5-(CF3)2C6H3]4]− commonly abbreviated as [BArF4]− and colloquially known as "BARF" was discovered; BARF is far less coordinating than hexafluorophosphate.
Applications
Practical uses of the hexafluorophosphate ion typically exploit one or more of the following properties: that it is a non-coordinating anionNon-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 as counterions for cationic metal complexes...
; that hexafluorophosphate compounds are typically soluble in organic solvents, particularly polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
ones, but have low solubility in aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
; or, that it has a high degree of stability, including resistance to both acidic and basic hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
.
Secondary batteries
The main commercial use of hexafluorophosphate is as its lithium salt, lithium hexafluorophosphateLithium hexafluorophosphate
Lithium hexafluorophosphate is an inorganic compound with the formula LiPF6. This white crystalline powder is used in commercial secondary batteries, an application that exploits its high solubility in nonpolar solvents. Specifically, solutions of lithium hexafluorophosphate in propylene carbonate...
. This salt in combination with dimethylcarbonate, is a common electrolyte in commercial secondary batteries 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 complexes within the electrolyte, the non-coordinating nature of the hexafluorophosphate ion is also a useful property for these applications.
Organometallic synthesis
Hexafluorophosphate salts are often included in organometallicOrganometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
syntheses
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
to provide an inert and non-coordinating counterion. Tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...
salts are a common alternative choice. One route to such compounds involves reactions with silver hexafluorophosphate
Silver hexafluorophosphate
Silver hexafluorophosphate, sometimes referred to "silver PF-6," is an inorganic compound with the formula AgPF6.-Uses and reactions:Silver hexafluorophospate is a commonly encountered reagent in inorganic and organometallic chemistry...
with the halide salt. Precipitation of insoluble silver halide salt 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
Ammonium hexafluorophosphate is a commonly available form of the hexafluorophosphate anion....
(NH4PF6) to aqueous solutions of many organic and inorganic salts gives solid precipitates of hexafluorophosphate salts. This method can have advantages over the silver hexafluorophosphate method in terms of expense and in systems where contamination with metal ions is strongly discouraged. As an example, the microwave synthesis
Microwave chemistry
Microwave chemistry is the science of applying microwave irradiation to chemical reactions. Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid...
of rhodicinium hexafluorophosphate
Rhodocene
Rhodocene, formally known as bisrhodium, is a chemical compound with the formula [Rh2]. Each molecule contains an atom of rhodium bound between two planar systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has covalent...
involves the reaction of cyclopentadiene
Cyclopentadiene
Cyclopentadiene is an organic compound with the formula C5H6. This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction...
and rhodium(III) chloride hydrate
Rhodium(III) chloride
Rhodium chloride refers to inorganic compounds with the formula RhCl3n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt...
in methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
. During the workup with methanolic ammonium hexafluorophosphate the product salt precipitates cleanly from the reaction mixture. The overall conversion equation is
- RhCl3.xH2O + 2 C5H6 + NH4PF6 → [(η5-C5H5)2Rh]PF6(s) + 2 HCl + NH4Cl + xH2O
Whilst the hexafluorophosphate ion is generally considered inert and hence a suitable counterion
Counterion
A counterion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium cation is the counterion for the chlorine anion and vice versa.In a charged transition metal complex, a simple A counterion is the ion that accompanies an ionic species in...
, solvolysis
Solvolysis
Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
reactions of the hexafluorophosphate ion are known. For example, the tris(solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
) rhodium
Organorhodium chemistry
Organorhodium chemistry is the chemistry of organometallic compounds containing a carbon to rhodium chemical bond, and the study of rhodium and rhodium compounds as catalysts in organic reactions....
complex [(η5-C5Me5)Rh(Me2CO)3](PF6)2 undergoes solvolysis when heated in acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
, forming a difluorophosphate-bridged complex [(η5-C5Me5)Rh(μ-OPF2O)3Rh(η5-C5Me5)]PF6.
Ionic liquids
Hexafluorophosphate organic salts that are soluble in organic solvents and ionic liquids are well known. The routes to such compounds are similar to those used for organometallic salts, such as anion exchange from the halide salt with silver hexafluorophosphate or use of ammonium hexafluorophosphate. Room temperature ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate1-Butyl-3-methylimidazolium hexafluorophosphate
1-Butyl-3-methylimidazolium hexafluorophosphate, also known as BMIM-PF6, is a viscous, colourless, hydrophobic and non-water soluble ionic liquid. Together with 1-butyl-3-methylimidazolium tetrafluoroborate, BMIM-BF4, it is one of the most widely studied ionic liquids and is commercially available...
(typically abbreviated as bmimPF6) have been prepared. The advantage of the anion exchange in favour of a non-coordinating anion is that the resulting ionic liquid has much higher thermally stability. The reason is that the possibility of decomposition of the imidazolium cation is decreased; 1-butyl-3-methylimidazolium chloride could decompose to N-methylimidazole
1-Methylimidazole
1-Methylimidazole or N-Methylimidazole is an aromatic heterocyclic organic compound with the formula CH3C3H3N2. Its N-methylation removes the possibility of tautomerization, which occurs in imidazole and many imidazole derivatives. 1-Methylimidazole maintains the pyridine-like nitrogen of...
and 1-chlorobutane
1-Chlorobutane
1-Chlorobutane is an alkyl halide with the chemical formula C4H9Cl. It reacts with lithium metal to give n-butyllithium:...
or to N-butylimidazole and chloromethane
Chloromethane
Chloromethane, also called methyl chloride, R-40 or HCC 40, is a chemical compound of the group of organic compounds called haloalkanes. It was once widely used as a refrigerant. It is a colorless extremely flammable gas with a minorly sweet odor, which is, however, detected at possibly toxic levels...
. Such decompositions are not possible for bmimPF6 However, thermal decompositions of hexafluorophosphate ionic liquids to generate hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
gas are known.
Inorganic synthesis
The hexafluorophosphate ion, being a large anion, can be useful in stabilising large cations. In the compound tetrakis(acetonitrile)copper(I) hexafluorophosphateTetrakis(acetonitrile)copper(I) hexafluorophosphate
Tetrakiscopper hexafluorophosphate is a coordination complex with the formula [Cu4]PF6. It is a colourless solid that is used in the synthesis of copper complexes.-Structure:...
, [Cu(CH3CN)4]PF6, the acetonitrile
Acetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s protect the copper(I) centre from oxidation to copper(II); combined with the lability of the complex, this makes for a suitable precursor in the non-aqueous syntheses of copper(I) compounds. However, the nearly linear arrangement of the 'arms' of this tetrahedral cation makes for a high degree of bulk. Consequently, the inclusion of a bulky counterion has a stabilising effect and a non-coordinating anion like hexafluorophosphate is ideal given the lability. The compound can be produced by the addition of hexafluorophosphoric acid to a suspension of copper(I) oxide
Copper(I) oxide
Copper oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper. This red-coloured solid is a component of some antifouling paints. The compound can appear either yellow or red, depending on the size of the particles, but both forms...
in acetonitrile:
- Cu2O + 2 HPF6 + 8 CH3CN → 2 [Cu(CH3CN)4]PF6 + H2O