hexafluorophosphate

File:Ferrocenium hexafluorophosphate.png

Hexafluorophosphate salts can be prepared by the reaction of phosphorus pentachloride and alkali or ammonium halide in a solution of hydrofluoric acid:{{cite book | last1 = Woyski | first1 = M. M. | title = Inorganic Syntheses | chapter = Hexafluophosphates of Sodium, Ammonium, and Potassium | journal = Inorg. Synth. | volume = 3 | pages = 111–117 | doi = 10.1002/9780470132340.ch29 | year = 1950| isbn = 9780470132340 }}

:{{chem2|PCl5 + MCl + 6 HF → M[PF6] + 6 HCl}}

Hexafluorophosphoric acid can be prepared by direct reaction of hydrogen fluoride with phosphorus pentafluoride.{{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 = 978-0-471-59668-4|page = 44}} It is a strong Brønsted acid that is typically generated in situ immediately before its use.

:{{chem2|PF5 + HF → H[PF6]}}

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

Several methods of quantitative analysis for the hexafluorophosphate ion have been developed. Tetraphenylarsonium chloride, {{chem2|[(C6H5)4As]Cl}}, has been used both for titrimetric{{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= Anal. Chem.|volume= 35|issue= 8|pages= 976–978|doi= 10.1021/ac60201a017}} and gravimetric{{cite journal |last1= Affsprung|first1= H. E.|last2= Archer|first2= V. S.|year= 1963|title= Gravimetric Determination of Hexafluorophosphate as Tetraphenylarsonium Hexafluorophosphate|journal= Anal. Chem.|volume= 35|issue= 12|pages= 1912–1913|doi= 10.1021/ac60205a036}} quantifications of hexafluorophosphate. Both of these determinations depend on the formation of tetraphenylarsonium hexafluorophosphate:

:{{chem2|[(C6H5)4As]+ + [PF6]- → [(C6H5)4As][PF6]}}

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

Hydrolysis is extremely slow under basic conditions.{{cite journal |last1= Ryss|first1= I. G.|last2= Tulchinskii|first2= V. B.|year= 1964|title= Kinetika Gidroliza Iona Geksaftorofosfata {{chem|PF|6|−}}|journal= Zh. Neorg. Khim. |volume= 9|issue= 4|pages= 836–840}} Acid-catalyzed hydrolysis to the phosphate ion is also slow.{{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}} Nonetheless, hexafluorophosphate is prone to decomposition with the release of hydrogen fluoride in ionic liquids.{{cite book

| title = Metal Catalysed Reactions in Ionic Liquids

| volume = 29

| series = Catalysis by Metal Complexes

| last1 = Dyson

| first1 = P. J.

| editor = Geldbach, T. J.

| publisher = Springer Science & Business

| year = 2005

| isbn = 1-4020-3914-X

| page = 27

}}

File:PF6partialcharge.png

Hexafluorophosphate is a common counteranion for cationic metal complexes. It is one of three widely used non-coordinating anions: hexafluorophosphate, tetrafluoroborate {{chem2|[BF4]−}}, and perchlorate {{chem2|ClO4−}}. Of these, the hexafluorophosphate ion has the least coordinating tendency.{{cite journal | last1 = Mayfield | first1 = H. G. | last2 = Bull | first2 = W. E. | title = Co-ordinating Tendencies of the Hexafluorophosphate Ion | year = 1971 | journal = J. Chem. Soc. A | issue = 14 | pages = 2279–2281 | doi = 10.1039/J19710002279}}

Hexafluorophosphate salts can be prepared by reactions of silver hexafluorophosphate 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 ({{chem2|[NH4][PF6]}}) to aqueous solutions of many organic and inorganic salts gives solid precipitates of hexafluorophosphate salts. One example is the synthesis of rhodocenium salts:{{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.|author-link2=D. Michael P. Mingos|last3 = Watson|first3 = M. J.|journal = J. Organomet. Chem.|volume = 368|issue = 3|pages = C43–C45|doi = 10.1016/0022-328X(89)85418-X|last4 = Watson|first4 = Michael J.}} The overall conversion equation is

:{{chem2|RhCl3*nH2O + 2 C5H6 + [NH4][PF6] → [(η^{5}\-C5H5)2Rh][PF6] + 2 HCl + [NH4]Cl + n H2O}}

Tetrakis(acetonitrile)copper(I) hexafluorophosphate is produced by the addition of hexafluorophosphoric acid to a suspension of copper(I) oxide in acetonitrile:{{cite journal | last1 = Kubas | first1 = G. J. | year = 1979 | title = Tetrakis(acetonitirile)copper(I) Hexaflurorophosphate | journal = Inorg. Synth. | volume = 19 | pages = 90–91 | doi = 10.1002/9780470132593.ch15 }}

:{{chem2|Cu2O + 2 H[PF6] + 8 CH3CN → 2 [Cu(CH3CN)4][PF6] + H2O}}

While the hexafluorophosphate ion is generally inert and hence a suitable counterion, its solvolysis can be induced by highly electrophilic metal centers. For example, the tris(solvento) rhodium complex {{chem2|[(η^{5}\-C5Me5)Rh(Me2CO)3][PF6]2}} undergoes solvolysis when heated in acetone, forming a difluorophosphate-bridged complex {{chem2|[(η^{5}\-C5Me5)Rh(μ\-OPF2O)3Rh(η^{5}\-C5Me5)][PF6]}}.{{cite journal |last1= Thompson|first1= S. J.|last2= Bailey|first2= P. M.|last3= White|first3= C.|author4=Peter Maitlis|author4-link=Peter Maitlis|year= 1976|title= Solvolysis of the Hexafluorophosphate Ion and the Structure of [Tris(μ-difluorophosphato)bis(penta-methylcyclopentadienylrhodium)] Hexafluorophosphate|journal= Angew. Chem. Int. Ed.|volume= 15|issue= 8|pages= 490–491|doi= 10.1002/anie.197604901}}{{cite journal |last2= Thompson|first2= S. J.|last1= White|first1= C.|author3= 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|volume= 134|issue= 3|pages= 319–325|doi= 10.1016/S0022-328X(00)93278-9}}

:Heating an acetone solution of [(η⁵-C₅Me₅)Rh(Me₂CO)₃](PF₆)₂ gives the difluorophosphate complex [(η⁵-C₅Me₅)Rh(μ-OPF₂O)₃Rh(η⁵-C₅Me₅)]⁺.

Practical uses of the hexafluorophosphate ion typically exploit one or more of the following properties: that it is a non-coordinating anion; 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.

The main commercial use of hexafluorophosphate is as its lithium salt, lithium hexafluorophosphate. This salt, in combination with dimethyl carbonate, 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.{{cite journal|title = Challenges for Rechargeable Li Batteries|first1 = J. B.|last1 = Goodenough|first2 = Y.|last2 = Kim|journal = Chem. Mater.|year = 2010|volume = 22|issue = 3|pages = 587–603|doi = 10.1021/cm901452z}} Since the lithium ions in these batteries are generally present as coordination complexes within the electrolyte,{{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|access-date = 11 June 2010|archive-url=https://web.archive.org/web/20110626215943/http://www.tek.com/Measurement/Service/msds/01914600.pdf|archive-date = 26 June 2011|url-status = dead}} the non-coordinating nature of the hexafluorophosphate ion is also a useful property for these applications.

Room temperature ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate (typically abbreviated as bmimPF₆) have been prepared.{{Cite journal| author3 = Alan R. Kennedy| author2 = John D. Holbrey| last1 = Gordon| author4 = Kenneth R. Seddon| issue = 12 | first1 = C. M.| title = Ionic liquid crystals: hexafluorophosphate salts| pages = 2627–2636| year = 1998 | doi = 10.1039/a806169f| journal = Journal of Materials Chemistry| volume = 8}} 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. Such decompositions are not possible for bmimPF₆. However, thermal decompositions of hexafluorophosphate ionic liquids to generate hydrogen fluoride gas are known.

:File:BMIM.png

{{Reflist|2}}{{Hexafluorophosphates}}

*

Category:Non-coordinating anions