Tetrachloronickelate is the metal complex with the formula [NiCl<sub>4</sub>]<sup>2âÂÂ</sup>. 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(H<sub>2</sub>O)<sub>5</sub>Cl]<sup>+</sup>. 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 (R<sub>3</sub>NH)<sub>2</sub>[NiCl<sub>4</sub>] are often thermochromic (R = Me, Et, Pr). Near room temperature, these salts are yellow, but these solids become blue when heated to near 70 ðC. The bright blue color is characteristic of tetrahedral [NiCl<sub>4</sub>]<sup>2âÂÂ</sup>, 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.
The blue colour due to the tetrachloronickelate ion was first observed in 1944 when Remy and Meyer melted caesium chloride and caesium nickel trichloride together.