Hexafluorophosphate's an anion with chemical formula of [PF6]−. As a non-coordinating anion, it's a poor nucleophile. Halide anions are easily replaced with [PF6]− with silver hexafluorophosphate. The precipitation of insoluble silver halide helps drive this reaction to completion. Some room temperature ionic liquids such as bmimPF6 use [PF6]− as a counterion. Also, many imidazolium and formamidinium hexafluorophosphate salts are insoluble in water. The addition of ammonium h… (More on Hexafluorophosphate)