Several methods of quantitative analysis for the hexafluorophosphate ion have been developed. Tetraphenylarsonium chloride, [(C6H5)4As]Cl, has been used both for titrimetric[6] and gravimetric[7] quantifications of hexafluorophosphate. Both of these determinations depend on the formation of tetraphenylarsonium hexafluorophosphate:
Hydrolysis is extremely slow under basic conditions.[9] Acid-catalyzed hydrolysis to the phosphate ion is also slow.[10] Nonetheless, hexafluorophosphate is prone to decomposition with the release of hydrogen fluoride in ionic liquids.[11]
Hexafluorophosphate bears a partial (delocalized) negative charge on the fluoride centers.
Hexafluorophosphate is a common counteranion for cationicmetal complexes. It is one of three widely used non-coordinating anions: hexafluorophosphate, tetrafluoroborate[BF4]−, and perchlorateClO−4. Of these, the hexafluorophosphate ion has the least coordinating tendency.[12]
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 ([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:[13] The overall conversion equation is
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 [(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].[15][16]
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.[17] Since the lithium ions in these batteries are generally present as coordination complexes within the electrolyte,[18] 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.[19] 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.[11]
^Davies, J. A. (1996). Synthetic Coordination Chemistry: Principles and Practice. World Scientific. p. 165. ISBN981-02-2084-7.
^Constant, S.; Lacour, J. (2005). J.-P. Majoral (ed.). New Trends in Hexacoordinated Phosphorus Chemistry. New Aspects in Phosphorus Chemistry. Vol. 5. Springer. p. 3. ISBN3-540-22498-X.
^Molnar, A.; Surya Prakash, G. K.; Sommer, J. (2009). Superacid Chemistry (2nd ed.). Wiley-Interscience. p. 44. ISBN978-0-471-59668-4.
^Affsprung, H. E.; Archer, V. S. (1963). "Determination of Hexafluorophosphate by Amperometric Titration with Tetraphenylarsonium Chloride". Anal. Chem.35 (8): 976–978. doi:10.1021/ac60201a017.
^Affsprung, H. E.; Archer, V. S. (1963). "Gravimetric Determination of Hexafluorophosphate as Tetraphenylarsonium Hexafluorophosphate". Anal. Chem. 35 (12): 1912–1913. doi:10.1021/ac60205a036.
^Archer, V. S.; Doolittle, F. G. (1967). "Spectrophotometric Determination of Hexafluorophosphate with Ferroin". Anal. Chem. 39 (3): 371–373. doi:10.1021/ac60247a035.
^Ryss, I. G.; Tulchinskii, V. B. (1964). "Kinetika Gidroliza Iona Geksaftorofosfata PF− 6". Zh. Neorg. Khim. 9 (4): 836–840.
^ abDyson, P. J. (2005). Geldbach, T. J. (ed.). Metal Catalysed Reactions in Ionic Liquids. Catalysis by Metal Complexes. Vol. 29. Springer Science & Business. p. 27. ISBN1-4020-3914-X.
^Baghurst, D. R.; Mingos, D. M. P.; Watson, M. J.; Watson, Michael J. (1989). "Application of Microwave Dielectric Loss Heating Effects for the Rapid and Convenient Synthesis of Organometallic Compounds". J. Organomet. Chem.368 (3): C43–C45. doi:10.1016/0022-328X(89)85418-X.
^Thompson, S. J.; Bailey, P. M.; White, C.; Peter Maitlis (1976). "Solvolysis of the Hexafluorophosphate Ion and the Structure of [Tris(μ-difluorophosphato)bis(penta-methylcyclopentadienylrhodium)] Hexafluorophosphate". Angew. Chem. Int. Ed.15 (8): 490–491. doi:10.1002/anie.197604901.
^White, C.; Thompson, S. J.; Peter Maitlis (1977). "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 of Organometallic Chemistry. 134 (3): 319–325. doi:10.1016/S0022-328X(00)93278-9.
^Gordon, C. M.; John D. Holbrey; Alan R. Kennedy; Kenneth R. Seddon (1998). "Ionic liquid crystals: hexafluorophosphate salts". Journal of Materials Chemistry. 8 (12): 2627–2636. doi:10.1039/a806169f.
2 gives the difluorophosphate complex [(η5-C5Me5)Rh(μ-OPF2O)3Rh(η5-C5Me5)]+.](http://en.wikipedia.org/wiki/File:Partial_solvolysis_of_hexafluorophospate.PNG)
