Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this anion, phosphorus has a valence of 5. Being poorly nucleophilic, hexafluorophosphate is classified as a non-coordinating anion. 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. It is a strong Brønsted acid that is typically generated in situ 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 ion is also slow. Nonetheless, hexafluorophosphate is prone to decomposition with the release of hydrogen fluoride in ionic liquids. Organometallic and inorganic synthesis Hexafluorophosphate is a common counteranion for cationic metal complexes. It is one of three widely used non-coordinating anions: hexafluorophosphate, tetrafluoroborate , and perchlorate . Of these, the hexafluorophosphate ion has the least coordinating tendency. 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 () 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 in acetonitrile: Hydrolysis of hexafluorophosphate complexes 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 undergoes solvolysis when heated in acetone, 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; that hexafluorophosphate compounds