Tetrachloronickelate
Tetrachloronickelate is the metal complex with the formula [NiCl4]2−. Salts of the complex are available with a variety of cations, but a common one is tetraethylammonium.
When concentrated lithium chloride and nickel chloride solution in water is mixed, only a pentaaquachloro complex is formed: [Ni(H2O)5Cl]+. 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.
Organic ammonium salts of the type (R3NH)2[NiCl4] are often thermochromic (R = Me, Et, Pr). Near room temperature, these salts are yellow , but these solids become blue when heated to near 70 °C. The bright blue color is characteristic of tetrahedral [NiCl4]2−, the intensity being a consequence of the Laporte selection rule. The yellow color results from a polymer consisting of octahedral Ni centers. The corresponding tetrabromonickelates are also thermochromic with a lower transition temperatures.