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.[1]

Sample of (Et4N)2NiCl4.
A tetrahedral tetrachloronickelate complex ion

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 salt [(n-C3H7)3NH]2NiCl4, is a blue ionic liquid at room temperature.[3] 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 cesium chloride and cesium nickel trichloride together.[4]

Compounds
formula name Ni Ox structure Remarks references
Cs2NiCl4 cesium tetrachloronickelate 2 blue stable only over 70° [4]
[(CH3)4N]2NiCl4 Tetramethylammonium tetrachloronickelate dark blue [5]
[(C2H5)4N]2NiCl4 Tetraethylammonium tetrachloronickelate 2 [6][4]
[H2NN(CH3)3]2NiCl4 1,1,1-trimethylhydrazinium tetrachloronickelate 2 blue, only stable over 145 °C, under this is yellow [7]
[(C6H5)4As]2NiCl4 bis-tetraphenylarsonium tetrachloronickelate blue melts 199.5° [5]
[(C6H5)3CH3As]2NiCl4 bis-triphenylmethylarsonium tetrachloronickelate blue [4]
[C2mim]2NiCl4 1-ethyl-3-methylimidazolium tetrachloronickelate(II) 2 tetragonal I41/a a=14.112 c=19.436 V=3871.1 Z=8 density=1.47 MW=422.84 melt 92° [8][9]
[C4mim]2NiCl4 1-butyl-3-methylimidazolium tetrachloronickelate(II) 2 dark blue melt 56° [8]
[C5mim]2NiCl4 1-pentyl-3-methylimidazolium tetrachloronickelate(II) 2 dark blue liquid [8]
[C6mim]2NiCl4 1-hexyl-3-methylimidazolium tetrachloronickelate(II) 2 dark blue liquid [8]
[C7mim]2NiCl4 1-heptyl-3-methylimidazolium tetrachloronickelate(II) 2 dark blue liquid [8]
[C8mim]2NiCl4 1-octyl-3-methylimidazolium tetrachloronickelate(II) 2 dark blue liquid [8]
[NH3CH2CH2NH3]NiCl4 ethylenediammonium tetrachloronickelate(II) 2 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]

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