Hexafluorophosphate is an anion with chemical formula of . As a non-coordinating anion, it's a poor nucleophile. It is prone to decomposition with the release of HF. Halide anions are easily replaced with by reaction with silver hexafluorophosphate. The precipitation of insoluble silver halide helps drive this reaction to completion. Some room temperature ionic liquids, such as bmimPF6, use as a counterion. Also, many imidazolium and formamidinium hexafluorophosphate salts are insoluble in wate… (More on Hexafluorophosphate)