ammoniacal PF6 chloride mercuric monofluoride nitrate triflate Grignard Ag2S selenide chlorides AgBr reagent dichloride tetrafluoride anions hydroxide telluride alkyne fluorides acetylides iodomethane Triphenylphosphine anhydrous CoCl2 persulfate chromate alkynes hexacarbonyl diethyl Nb2O5 cupellation AgI alkenes argyros argyrodite alkali