Tetrachloronickelate Explained

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] 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.

Compounds

formula	name	structure	Remarks	references
Cs2NiCl4	caesium tetrachloronickelate		blue, stable only over 70°
[(CH<sub>3</sub>)<sub>4</sub>N]2NiCl4	Tetramethylammonium tetrachloronickelate		dark blue	^[4]
[(C<sub>2</sub>H<sub>5</sub>)<sub>4</sub>N]2NiCl4	Tetraethylammonium tetrachloronickelate			^[5] ^[6]
[H<sub>2</sub>NN(CH<sub>3</sub>)<sub>3</sub>]2NiCl4	1,1,1-trimethylhydrazinium tetrachloronickelate		blue, only stable over 145 °C, under this is yellow	^[7]
[(C<sub>6</sub>H<sub>5</sub>)<sub>4</sub>As]2NiCl4	bis-tetraphenylarsonium tetrachloronickelate		blue, melts at 199.5°C
[(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>CH<sub>3</sub>As]2NiCl4	bis-triphenylmethylarsonium tetrachloronickelate		blue
[C<sub>2</sub>mim]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]
[C<sub>4</sub>mim]2NiCl4	1-butyl-3-methylimidazolium tetrachloronickelate(II)	dark blue, melts at 56°C		^[9]
[C<sub>5</sub>mim]2NiCl4	1-pentyl-3-methylimidazolium tetrachloronickelate(II)	dark blue liquid
[C<sub>6</sub>mim]2NiCl4	1-hexyl-3-methylimidazolium tetrachloronickelate(II)	dark blue liquid
[C<sub>7</sub>mim]2NiCl4	1-heptyl-3-methylimidazolium tetrachloronickelate(II)	dark blue liquid
[C<sub>8</sub>mim]2NiCl4	1-octyl-3-methylimidazolium tetrachloronickelate(II)	dark blue liquid
[NH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>NH<sub>3</sub>]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]

Related substances

• tetrabromonickelates

Notes and References

1. Book: Naida S. Gill . F. B. Taylor . Tetrahalo Complexes of Dipositive Metals in the First Transition Series . 1967 . 9 . 136–142 . 10.1002/9780470132401.ch37 . Inorganic Syntheses . 978-0-470-13240-1.
2. Florence. T. M.. Farrar. Yvonne J.. Liquid-liquid extraction of nickel with long-chain amines from aqueous and nonaqueous halide media. Analytical Chemistry. July 1968. 40. 8. 1200–1206. 10.1021/ac60264a010.
3. Ferraro. John R.. Sheren. Anne T.. Some new thermochromic complexes of Nickel(II) of the type [R/sub x/NH/sub 4-x/]2NiCl4. Inorganic Chemistry. 17. 9. 2498–2502. 3 February 1978. 10.1021/ic50187a032.
4. Cotton. F. A.. Faut. O. D.. Goodgame. D. M. L.. Preparation, Spectra and Electronic Structures of Tetrahedral Nickel(II) Complexes Containing Triphenylphosphine and Halide Ions as Ligands. Journal of the American Chemical Society. January 1961. 83. 2. 344–351. 10.1021/ja01463a021.
5. Inman. Guy W.. Hatfield. William E.. Jones. Edwin R.. The magnetic properties of tetraethylammonium tetrachloronickelate(II) and tetraethylammonium tetrabromonickelate(II). Inorganic and Nuclear Chemistry Letters. August 1971. 7. 8. 721–723. 10.1016/0020-1650(71)80078-8.
6. Gill. Naida S.. Nyholm. R. S.. 802. Complex halides of the transition metals. Part I. Tetrahedral nickel complexes. Journal of the Chemical Society (Resumed). 1959. 3997. 10.1039/JR9590003997.
7. Goedken. V. L.. Vallarino. L. M.. Quagliano. J. V.. Cationic ligands. Coordination of the 1,1,1-trimethylhydrazinium cation to nickel(II). Inorganic Chemistry. December 1971. 10. 12. 2682–2685. 10.1021/ic50106a011.
8. Hitchcock. Peter B.. Seddon. Kenneth R.. Welton. Thomas. Hydrogen-bond acceptor abilities of tetrachlorometalate(II) complexes in ionic liquids. Journal of the Chemical Society, Dalton Transactions. 1993. 17. 2639. 10.1039/DT9930002639.
9. Meredith. M. Brett. McMillen. C. Heather. Goodman. Jonathan T.. Hanusa. Timothy P.. Ambient temperature imidazolium-based ionic liquids with tetrachloronickelate(II) anions. Polyhedron. August 2009. 28. 12. 2355–2358. 10.1016/j.poly.2009.04.037.
10. Skaarup. Steen. Berg. Rolf Willestofte. Structural properties and vibrational spectra of the ethylene-diammonium family of perovskite layer-type crystals: [NH3CH2CH2NH3] [MCl4], M = Ni, Pd, Cu, Cd, Mn. Journal of Solid State Chemistry. September 1978. 26. 1. 59–67. 10.1016/0022-4596(78)90133-0. 1978JSSCh..26...59S .