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 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]

Related substances

• tetrabromonickelates