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 conve ...
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 a ...
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 th ...
with
sulfur hexafluoride, , 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.
Hexafluo ...
dianion, , and
hexafluoroantimonate . 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
Hexafluorophosphate salts can be prepared by the reaction of
phosphorus pentachloride and alkali or ammonium halide in a solution of
hydrofluoric acid:
:
Hexafluorophosphoric acid can be prepared by direct reaction of
hydrogen fluoride 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 quantifications of hexafluorophosphate. Both of these determinations depend on the formation of tetraphenylarsonium hexafluorophosphate:
:
Hexafluorophosphate can also be determined
spectrophotometrically with
ferroin.
Reactions
Hydrolysis is extremely slow under basic conditions. Acid-catalyzed
hydrolysis 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 phosph ...
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 . 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 () 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
:
Tetrakis(acetonitrile)copper(I) hexafluorophosphate 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, 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 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 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.
Secondary batteries
The main commercial use of hexafluorophosphate is as its lithium salt,
lithium hexafluorophosphate. 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 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. ...
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 gas are known.
:
References
{{Hexafluorophosphates
*
Non-coordinating anions