Nickel(II) fluoride
Nickel(II) fluoride is the chemical compound with the formula NiF. Its is an ionic compound of nickel and fluorine and forms yellowish to green tetragonal crystals. Unlike many fluorides, NiF is stable in air. Nickel(II) fluoride is also produced when nickel metal is exposed to fluorine. In fact, NiF comprises the passivating surface that forms on nickel alloys (e.g. monel) in the presence of hydrogen fluoride or elemental fluorine. For this reason nickel and its alloys are suitable materials for storage and transport these fluorine and related fluorinating agents. NiF is also used as a catalyst for the  synthesis of chlorine pentafluoride. Preparation and structure. NiF is prepared by treatment of anhydrous nickel(II) chloride with fluorine at 350 °C: The corresponding reaction of cobalt(II) chloride results in oxidation of the cobalt, whereas nickel remains in the +2 oxidation state after fluorination because its +3 oxidation state is less stable. Chloride is more easily oxidized than nickel(II). This is a typical halogen displacement reaction, where a halogen plus a less active halide makes the less active halogen and the more active halide. Like some other metal difluorides, NiF crystallizes in the rutile structure, which features octahedral Ni centers  and planar fluorides. Reactions. A melt of NiF and KF reacts to give successively potassium trifluoronickelate and potassium tetrafluoronickelate: The structure of this material is closely related to some superconducting oxide materials. Nickel(II) fluoride reacts with strong bases to give nickel(II) hydroxide: