Wikipedia
Tetrachloronickelates are a class of chemical compounds containing anions consisting of a nickel atom surrounded by four chloride ions. [NiCl]
When concentrated lithium chloride and nickel chloride solution in water is mixed, only a pentaaquachloro complex is formed: [Ni(HO)Cl]. However in other organic solvents, or molten salts the tetrachloronickelate ion can form. Nickel can be separated from such a solution in water or methanol, by partitioning it into a cyclohexane solution of amines.
Substituted ammonia tetrachloronickelates can be thermochromic. At room temperature they can be in a polymeric yellow coloured form, but when heated become bright blue, as the form changes to a tetrahedral arrangement of chlorine around the nickel atom. Yet other amine salts of tetrachloronickelate, such as [(NCH)NH]NiCl, are blue liquids at room temperature. The trimethylammonium and diethylammonium salts have thermochromic temperatures at 70 °C. The corresponding tetrabromonickelates are also thermochromic with a transition at lower temperatures.
Some tetrachloronickelates can be ionic liquids at room temperature.