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.[1]
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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.[2]
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.[3]
History
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.[4]
Compounds
| formula | name | structure | Remarks | references |
|---|---|---|---|---|
| Cs2NiCl4 | caesium tetrachloronickelate | blue, stable only over 70° | [4] | |
| [(CH3)4N]2NiCl4 | Tetramethylammonium tetrachloronickelate | dark blue | [5] | |
| [(C2H5)4N]2NiCl4 | Tetraethylammonium tetrachloronickelate | [6][4] | ||
| [H2NN(CH3)3]2NiCl4 | 1,1,1-trimethylhydrazinium tetrachloronickelate | blue, only stable over 145 °C, under this is yellow | [7] | |
| [(C6H5)4As]2NiCl4 | bis-tetraphenylarsonium tetrachloronickelate | blue, melts at 199.5°C | [5] | |
| [(C6H5)3CH3As]2NiCl4 | bis-triphenylmethylarsonium tetrachloronickelate | blue | [4] | |
| [C2mim]2NiCl4 | 1-ethyl-3-methylimidazolium tetrachloronickelate(II) | tetragonal I41/a a=14.112 c=19.436 V=3871.1 Z=8 density=1.47 MW=422.84 | melts at 92° | [8][9] |
| [C4mim]2NiCl4 | 1-butyl-3-methylimidazolium tetrachloronickelate(II) | dark blue, melts at 56°C | [8] | |
| [C5mim]2NiCl4 | 1-pentyl-3-methylimidazolium tetrachloronickelate(II) | dark blue liquid | [8] | |
| [C6mim]2NiCl4 | 1-hexyl-3-methylimidazolium tetrachloronickelate(II) | dark blue liquid | [8] | |
| [C7mim]2NiCl4 | 1-heptyl-3-methylimidazolium tetrachloronickelate(II) | dark blue liquid | [8] | |
| [C8mim]2NiCl4 | 1-octyl-3-methylimidazolium tetrachloronickelate(II) | dark blue liquid | [8] | |
| [NH3CH2CH2NH3]NiCl4 | ethylenediammonium tetrachloronickelate(II) | perovskite layered, space group P21/c a=8.441, b=6.995, c=6.943, β=92.925 Z=2 | brown, isostructural with Mn, Cu, Pd, and Cd compounds | [10] |