Hexafluorophosphate: Difference between revisions

| ImageFileL1 = PF6.png

| ImageFileL1_Ref = {{chemboximage|correct|??}}

| ImageSizeL1 = 121

| ImageNameL1 = Stereo skeletal formula of hexafluorophosphate

| ImageFileR1_Ref = {{chemboximage|correct|??}}

| ImageSizeR1 = 121

| ImageNameR1 = Spacefill model of hexafluorophosphate

| Section1 = {{Chembox Identifiers

| PubChem = 9886

| PubChem_Ref = {{Pubchemcite|correct|pubchem}}

| ChemSpiderID = 9502

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChEBI = 30201

| ChEMBL = 181124

| ChEMBL_Ref = {{ebicite|correct|EBI}}

| Gmelin = 2704

| SMILES = F[P-](F)(F)(F)(F)F

| StdInChI = 1S/F6P/c1-7(2,3,4,5)6/q-1

| StdInChIKey = LJQLCJWAZJINEB-UHFFFAOYSA-N

| Section2 = {{Chembox Properties

| Formula = F₆P^-

| MolarMass = 144.964181 g mol^-1

| ExactMass = 144.964180742 g mol^-1

'''Hexafluorophosphate''' is an anion with chemical formula of {{chem|PF|6|-}}. This [[Octahedral molecular geometry|octahedral]] species is [[isoelectronic]] with [[sulfur hexafluoride]], SF₆, and is valence isoelectronic with the highly stable [[superacid]] anion [[fluoroantimonic acid|fluoroantimonate]] {{chem|SbF|6|-}}. As a [[non-coordinating anion]],<ref>{{cite doi|10.1016/j.inoche.2007.05.008}}</ref><ref name = "Davies_1996">{{cite book

| title = Synthetic Coordination Chemistry: Principles and Practice

| last1 = Davies

| first1 = J. A.

| publisher = World Scientific

| year = 1996

| isbn = 9810220847

| page = 165

}}</ref><ref name = "Constant_2005">{{cite book

| series = New Aspects in Phosphorus Chemistry

| title = New Trends in Hexacoordinated Phosphorus Chemistry

| volume = 5

| last1 = Constant

| first1 = S.

| last2 = Lacour

| first2 = J.

| editor = Majoral, J.-P.

| publisher = Springer

| year = 2005

| isbn = 354022498X

| page = 3

}}</ref> it is a poor [[nucleophile]]. It is prone to decomposition with the release of [[hydrogen fluoride]] in [[ionic liquid]]s<ref name = "Dyson_book">{{cite book

| publisher = Springer Science &amp; Business

| isbn = 140203914X

}}</ref> but is generally extremely stable in solution. [[Hydrolysis]] to the [[phosphate]] ion is very slow even in concentrated [[acid]] with warming,<ref>{{cite journal |last1= Gebala|first1= A. E.|last2= Jones|first2= M. M.|year= 1969|title= The Acid Catalyzed Hydrolysis of Hexafluorophosphate|journal= [[J. Inorg. Nucl. Chem.]] |volume= 31|issue= 3|pages= 771–776|doi= 10.1016/0022-1902(69)80024-2}}</ref> and even slower under basic conditions.<ref>{{cite journal |last1= Ryss|first1= I. G.|last2= Tulchinskii|first2= V. B.|year= 1964|title= Kinetika Gidroliza Iona Geksaftorofosfata PF₆^-|journal= [[Zh. Neorg. Khim.]] |volume= 9|issue= 4|pages= 836–840}}</ref>

==Synthesis==

Hexafluorophosphate salts can be prepared by the reaction of [[phosphorus pentachloride]] and alkali or ammonium halide in a solution of [[hydrofluoric acid]]:<ref>{{cite journal | last1 = Woyski | first1 = M. M. | title = Hexafluorophosphates of Sodium, Ammonium, and Potassium | journal = [[Inorg. Synth.]] | volume = 3 | pages = 111–117 | doi = 10.1002/9780470132340.ch29 | year = 1950 | last2 = Shenk | first2 = W. J. | last3 = Pellon | first3 = E. R.}}</ref>

:PCl₅ + MCl + 6 HF &rarr; MPF₆ + 6 HCl

[[Hexafluorophosphoric acid]] can be prepared by direct reaction of [[hydrogen fluoride]] with [[phosphorus pentafluoride]].<ref>{{cite book|title = Superacid Chemistry|last1 = Molnar|first1 = A.|last2 = Surya Prakash|first2 = G. K.|last3 = Sommer|first3 = J.|edition = 2nd|publisher = Wiley-Interscience|year = 2009|isbn = 047159668X|page = 44}}</ref> It is a strong [[Brønsted–Lowry acid-base theory|Brønsted acid]] that is typically generated ''[[In situ#Chemistry and chemical engineering|in situ]]'' immediately prior to its use.

:PF₅ + HF &rarr; HPF₆

These reactions require specialized equipment to safely handle the hazards associated with hydrofluoric acid solution and hydrogen fluoride gas.

==Quantitative analysis==

[[Quantitative analysis (chemistry)|Quantitative analysis]] 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 [[titration|titrimetric]]<ref>{{cite journal |last1= Affsprung|first1= H. E.|last2= Archer|first2= V. S.|year= 1963|title= Determination of Hexafluorophosphate by Amperometric Titration with Tetraphenylarsonium Chloride|journal= [[Analytical Chemistry (journal)|Anal. Chem.]]|volume= 35|issue= 8|pages= 976–978|doi= 10.1021/ac60201a017}}</ref> and [[gravimetric analysis|gravimetric]]<ref>{{cite journal |last1= Affsprung|first1= H. E.|last2= Archer|first2= V. S.|year= 1963|title= Gravimetric Determination of Hexafluorophosphate as Tetraphenylarsonium Hexafluorophosphate|journal= [[Analytical Chemistry (journal)|Anal. Chem.]]|volume= 35|issue= 12|pages= 1912–1913|doi= 10.1021/ac60205a036}}</ref> quantifications of hexafluorophosphate. Both of these determinations depend on the formation of tetraphenylarsonium hexafluorophosphate:

:[(C₆H₅)₄As]⁺ + PF₆^&minus; &rarr; [(C₆H₅)₄As]PF₆

Hexafluorophosphate can also be determined [[Spectrophotometry|spectrophotometrically]] with [[ferroin]].<ref>{{cite journal |last2= Doolittle|first2= F. G.|last1= Archer|first1= V. S.|year= 1967|title= Spectrophotometric Determination of Hexafluorophosphate with Ferroin|journal= [[Analytical Chemistry (journal)|Anal. Chem.]]|volume= 39|issue= 3|pages= 371–373|doi= 10.1021/ac60247a035}}</ref>

==Properties==

[[Non-coordinating anion]]s are anions that interact only weakly with cations, a useful property when studying highly [[electrophilic]] cations.<ref>{{cite journal | last1 = Krossing | first1 = I. | last2 = Raabe | first2 = I. | title = Noncoordinating Anions - Fact or Fiction? A Survey of Likely Candidates | year = 2004 | journal = [[Angewandte Chemie|Angew. Chem. Int. Ed.]] | volume = 43 | pmid = 15083452 | issue = 16 | pages = 2066–2090 | doi = 10.1002/anie.200300620}}</ref> In [[coordination complex|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.<ref name = "Davies_1996" /><ref name = "Constant_2005" /> Three widely used non-coordinating anions are hexafluorophosphate, [[tetrafluoroborate]] {{chem|BF|4|&minus;}}, and [[perchlorate]] {{chem|ClO|4|&minus;}}; of these, the hexafluorophosphate ion has the least coordinating ability<ref>{{cite journal | last1 = Mayfield | first1 = H. G. | last2 = Bull | first2 = W. E. | title = Co-ordinating Tendencies of the Hexafluorophosphate Ion | year = 1971 | journal = [[Journal of the Chemical Society|J. Chem. Soc. A]] | issue = 14 | pages = 2279–2281 | doi = 10.1039/J19710002279}}</ref> and it is deliberately used for this property. In the 1990s, a new non-coordinating anion, [B[3,5-(CF₃)₂C₆H₃]₄]^&minus; commonly abbreviated as [BAr^F₄]⁻ and colloquially known as "BARF"<ref>{{Cite journal | last1 = Yakelis | first1 = N. A. | first2 = R. G. | last2 = Bergman | title = Safe Preparation and Purification of Sodium Tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBArF₂₄): Reliable and Sensitive Analysis of Water in Solutions of Fluorinated Tetraarylborates | journal = [[Organometallics]] | year = 2005 | volume = 24 | issue = 14 | pmid = 19079785 | pmc = 2600718 | pages = 3579–3581 | doi = 10.1021/om0501428}}</ref> was discovered; BARF is far less coordinating than hexafluorophosphate.<ref>{{Cite journal | last1 = Brookhart | first1 = M. | last2 = Grant | first2 = B. | last3 = Volpe, Jr. | first3 = A. F. | title = [(3,5-(CF₃)₂C₆H₃)₄B]^&minus;[H(OEt₂)₂]⁺: A Convenient Reagent for Generation and Stabilization of Cationic, Highly Electrophilic Organometallic Complexes| journal = [[Organometallics]] | year = 1992 | volume = 11 | pages = 3920–3922 | doi = 10.1021/om00059a071}}</ref>

The main commercial use of hexafluorophosphate is as its lithium salt, [[lithium hexafluorophosphate]]. This salt in combination with [[dimethylcarbonate]], is a common electrolyte in commercial [[secondary battery|secondary batteries]] such as [[lithium-ion battery|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.<ref>{{cite journal|title = Challenges for Rechargeable Li Batteries|first1 = J. B.|last1 = Goodenough|first2 = Y.|last2 = Kim|journal = [[Chemistry of Materials|Chem. Mater.]]|year = 2010|volume = 22|issue = 3|pages = 587–603|doi = 10.1021/cm901452z}}</ref> Since the lithium ions in these batteries are generally present as [[coordination complex]]es within the electrolyte,<ref>{{cite web|url = http://www.tek.com/Measurement/Service/msds/01914600.pdf|title = MSDS: National Power Corp Lithium Ion Batteries|work = tek.com|publisher = Tektronix Inc.|date = 7 May 2004|accessdate = 11 June 2010}}</ref> the non-coordinating nature of the hexafluorophosphate ion is also a useful property for these applications.

===Organometallic synthesis===

Hexafluorophosphate salts are often included in [[organometallic chemistry|organometallic]] [[chemical synthesis|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 (chemistry)|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 chemistry|microwave synthesis]]<ref>{{cite journal|title = Design and Application of a Reflux Modification for the Synthesis of Organometallic Compounds Using Microwave Dielectric Loss Heating Effects|year = 1990|last1 = Baghurst|first1 = D. R.|last2 = Mingos|first2 = D. M. P.|journal = [[Journal of Organometallic Chemistry|J. Organomet. Chem.]]|volume = 384|issue = 3|pages = C57–C60|url = |doi = 10.1016/0022-328X(90)87135-Z}}</ref> of [[rhodocene|rhodicinium hexafluorophosphate]] involves the reaction of [[cyclopentadiene]] and [[rhodium(III) chloride|rhodium(III) chloride hydrate]] in [[methanol]]. During the [[workup (chemistry)|workup]] with methanolic ammonium hexafluorophosphate the product salt precipitates cleanly from the reaction mixture.<ref>{{cite journal|title = Application of Microwave Dielectric Loss Heating Effects for the Rapid and Convenient Synthesis of Organometallic Compounds|year = 1989|last1 = Baghurst|first1 = D. R.|last2 = Mingos|first2 = D. M. P.|last3 = Watson|first3 = M. J.|journal = [[Journal of Organometallic Chemistry|J. Organomet. Chem.]]|volume = 368|issue = 3|pages = C43–C45|url = |doi = 10.1016/0022-328X(89)85418-X}}</ref> The overall conversion equation is

:RhCl₃.''x''H₂O + 2 C₅H₆ + NH₄PF₆ &rarr; [(&eta;⁵-C₅H₅)₂Rh]PF₆(s) + 2 HCl + NH₄Cl + ''x''H₂O

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]]) [[organorhodium chemistry|rhodium]] complex [(η⁵-C₅[[methyl|Me]]₅)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.<ref>{{cite journal |last1= Thompson|first1= S. J.|last2= Bailey|first2= P. M.|last3= White|first3= C.|last4= Maitlis|first4= P. M.|authorlink4=Peter Maitlis|year= 1976|title= Solvolysis of the Hexafluorophosphate Ion and the Structure of [Tris(μ-difluorophosphato)bis(penta-methylcyclopentadienylrhodium)] Hexafluorophosphate|journal= [[Angewandte Chemie|Angew. Chem. Int. Ed.]]|volume= 15|issue= 8|pages= 490–491|doi= 10.1002/anie.197604901}}</ref><ref>{{cite journal |last2= Thompson|first2= S. J.|last1= White|first1= C.|last3= Maitlis|first3= P. M.|authorlink3= Peter Maitlis|year= 1977|title= Pentamethylcyclopentadienyl-rhodium and -iridium Complexes '''''XIV.''''' The Solvolysis of Coordinated Acetone Solvent Species to Tris(μ-difluorophosphato)bis[η⁵-pentamethylcyclopentadienylrhodium(III)] Hexafluorophosphate, to the η⁵-(2,4-dimethyl-1-oxapenta-1,3-dienyl)(pentamethylcyclopentadienyl)iridium Cation, or to the η⁵-(2-hydroxy-4-methylpentadienyl)(η⁵-pentamethylcyclopentadienyl)iridium Cation|journal= [[Journal of Organometallic Chemistry|J. Organomet. Chem.]]|volume= 134|issue= 3|pages= 319–325|doi= 10.1016/S0022-328X(00)93278-9}}</ref>

:[[Image:Partial solvolysis of hexafluorophospate.PNG|Heating [(η⁵-C₅Me₅)Rh(Me₂CO)₃](PF₆)₂ in acetone results in the formation of the complex [(η⁵-C₅Me₅)Rh(μ-OPF₂O)₃Rh(η⁵-C₅Me₅)]PF₆]]

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 hexafluorophosphate|1-butyl-3-methylimidazolium hexafluorophosphate]] (typically abbreciated as bmimPF₆) have been prepared.<ref>{{cite doi|10.1039/a806169f}}</ref> 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 [[1-Methylimidazole|''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.<ref name = "Dyson_book" />

:[[Image:BMIM.png|300px|Preparation of [[1-Butyl-3-methylimidazolium hexafluorophosphate|1-butyl-3-methylimidazolium hexafluorophosphate]] from [[1-Methylimidazole|''N''-methylimidazole]] and [[1-chlorobutane]]]]

===Inorganic synthesis===

The hexafluorophosphate ion, being a large anion, can be useful in stabilising large cations. In the compound [[tetrakis(acetonitrile)copper(I) hexafluorophosphate]], [Cu(CH₃CN)₄]PF₆, the [[acetonitrile]] [[ligand]]s protect the copper(I) centre from oxidation to copper(II); combined with the [[labile|lability]] of the complex, this makes for a suitable precursor in the non-aqueous syntheses of copper(I) compounds.<ref name = "Kubas_1979">{{cite journal | last1 = Kubas | first1 = G. J. | year = 1979 | title = Tetrakis(acetonitirile)copper(I) Hexaflurorophosphate | journal = [[Inorganic Syntheses|Inorg. Synth.]] | volume = 19 | pages = 90–91 | doi = 10.1002/9780470132593.ch15 | last2 = Monzyk | first2 = B. | last3 = Crumblis | first3 = A. L.}}</ref> However, the nearly linear arrangement of the 'arms' of this tetrahedral cation makes for a high degree of bulk.<ref>{{cite journal | last1 = Csöregh | first1 = I. | last2 = Kierkegaard | first2 = P. | last3 = Norrestam | first3 = R. | year = 1975 | title = Copper(I) Tetraacetonitrile Perchlorate | journal = [[Acta Crystallographica|Acta Crystallogr. B]] | volume = 31 | pages = 314–317 | doi = 10.1107/S0567740875002634}}</ref> 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:<ref name = "Kubas_1979" />

:Cu₂O + 2 HPF₆ + 8 CH₃CN → 2 [Cu(CH₃CN)₄]PF₆ + H₂O

==See also==

* [[Non-coordinating anion]]

* [[Hexafluorophosphoric acid]]

{{Reflist|2}}

[[nl:Hexafluorfosfaat]]

[[ja:ヘキサフルオロリン酸塩]]