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Four cyano groups are arranged in a square in the tetracyanonickelate ion.

The cyanonickelates are a class of chemical compound containing anions consisting of nickel atoms, and cyanide groups. The most important of these are the tetracyanonickelates containing four cyanide groups per nickel. The tetracyanonickelates contain the [Ni(CN)₄]²⁻ anion. This can exist in solution or in solid salts. The ion has cyanide groups arranged in a square around the central nickel ion. The symmetry of the ion is D_4h. The distance from the nickel atom to the carbon is 1.87 Å, and the carbon-nitrogen distance is 1.16 Å.^[1] In their crystals, the tetracyanonickelate(II) anions are often arranged in a columnar structure (e.g. in K₂[Ni(CN)₄]^[2]). Tetracyanonickelate(II) can be oxidised electrochemically in solution to yield tetracyanonickelate(III) [Ni(CN)₄]⁻. [Ni(CN)₄]⁻ is unstable and Ni(III) oxidises the cyanide to cyanate OCN⁻.^[3] Tetracyanonickelate(III) can add two more cyanide groups to form hexacyanonickelate(III).^[3]

In combination with alkyldiamines, and other metal ions, tetracyanonickelate ions can form cage structure that can accommodate organic molecules. This is a Hofmann-diam-type clathrate.^[4]

If the cation is a very strong reducing agent, such as Yb²⁺, [Ni(CN)₄]²⁻ can be reduced to [Ni₂(CN)₆]⁴⁻ where nickel atom is in the +1 oxidation state.^[5]

formula	name	Ni Ox	structure	Remarks	references
Na₂[Ni(CN)₄]·3H₂O	disodium tetracyanonickelate(II)	2	triclinic a = 7.392 Å, b = 8.895 Å, c = 15.11 Å , α = 89.12, β= 87.46, γ = 84.54° Z=4 V=988 Å³ FW=262.81	orange; Ni(CN)₄ planes parallel	^[6]^[7]
K₂[Ni(CN)₄]·H₂O	potassium tetracyanonickelate(II)	2	monoclinic	orange-red; dehydrate at 110°	^[2]
K₂[Ni(CN)₄]	Potassium tetracyanonickelate	2	monoclinic a=4.294 Å, b=7.680 Å, c=13.02 Å, β=87°16′	orange-yellow; CN forms a square around Ni	^[2]
Ca[Ni(CN)₄]·5H₂O	calcium tetracyanonickelate	2	orthorhombic Pcab a=18.18 Å, b=18.86 Å, c=6.774 Å, Z=8, V=2195 Å³, density=1.774	CN forms distorted square around Ni; yellow when light polarized in elongation direction, colourless ⊥	^[8]
Co·2H₂O[Ni(CN)₄]·4H₂O	diaquacobalt tetracyanonickelate tetrahydrate	2	orthorhombic a=12.178 Å, b=13.885 Å, c=7.143 Å, V=1207.8 Z=4	orange MW=329.82	^[9]
Ni(NH₃)₂[Ni(CN)₄]·C₆H₆	Hofmann clathrate	2/2		benzene can be replaced by some other aromatic hydrocarbons; octahedral nickel can be replaced by Mn, Fe, Co, Cu, Zn or Cd. The square planar nickel can be replaced by Pd or Pt. Ammonia can be replaced by diamines and amines.	^[10]
Rb₂[Ni(CN)₄]·H₂O	rubidium tetracyanonickelate(II)	2	triclinic P1 a=8.602 Å, b=9.693 Å, c=12.006 Å, α = 92.621°, β= 94.263∘, γ =111.79° V=924.0 Å³	orange needles; Ni(CN)₄ planes parallel; water in 5% excess	^[7]
Sr[Ni(CN)₄]·5H₂O	strontium tetracyanonickelate(II)	2	monoclinic C2/m, a=10.356 Å, b=15.272 Å, c=7.133 Å, α=98.55°, V=1115.6 Å³	orange; Ni(CN)₄ planes parallel	^[7]
Cd·2H₂O[Ni(CN)₄]·4H₂O	diaquacadmium tetracyanonickelate tetrahydrate	2	orthorhombic Pnma, a = 12.393 Å, b = 14.278 Å, c = 7.427 Å, Z = 4, V=1314 Å³, density=1.937	MW=383.27	^[10]
Cs₂[Ni(CN)₄]·1.05H₂O	cesium tetracyanonickelate(II)	2	hexagonal, P61, a = 9.526 Å, c = 19.043 Å, V = 1496.5 Å³ Z=6	golden yellow; Ni(CN)₄ planes arranged in a spiral	^[11]
CsKNi(CN)₄	cesium potassium tetracyanonickelate	2	triclinic a= 7.421 Å, b= 8.626 Å, c= 9.364 Å, α= 60.64°,β= 70.88°, y= 70.88°, and Z= 2. den=2.55	orange	^[12]
Ba[Ni(CN)₄]·4H₂O	barium tetracyanonickelate(II)	2	monoclinic	red	^[13]
(Dimethylformamide)₄EuNi(CN)₄	europium(II) tetracyanonickelate	2	triclinic P1̄, a=8.902Å, b=10.947Å, c = 12.464Å, α = 82.99°, β = 86.86°, γ = 84.92°, Z = 2		^[5]
Tl₂[Ni(CN)₄]·1.05H₂O	thallium(I) tetracyanonickelate	2	monoclinic a=6.154 b=7.282 c=9.396 β=104.29 V=408.0 Z=2 density=4.652	bright yellow orange; chains of TlNi	^[12]
(UO₂)₂(dmso)₄(OH)₂[Ni(CN)₄]		2	monoclinic C2/c a=21.522 Å, b=10.2531 Å, c=13.3170 Å, β=111.943° V=2725.8 Å	yellow	^[14]
K₄[(CN)₃Ni-Ni(CN)₃]	potassium hexacyanodinickelate(I)	1			"Bellucci's salt"^[15]

References

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^ ^a ^b Wang, Yi Lai; Beach, Mark W.; Pappenhagen, Thomas L.; Margerum, Dale W. (November 1988). "Mixed-ligand complexes of tetracyanonickelate(III) and dynamic Jahn-Teller distortions of hexacyanonickelate(III)". Inorganic Chemistry. 27 (24): 4464–4472. doi:10.1021/ic00297a025.
^ Şenyel, Mustafa; Raci Sertbakan, T.; Kürkçüoğ, Güneş; Kasap, Ergün; Kantarci, Ziya (2001). "An Infrared Spectroscopic Study on the Hofmann-diam-type 1,12-Diaminododecanemetal(II) Tetracyanonickelate(II)-aromatic Guest Clathrates: M(H₂N(CH₂)₁₂NH₂)Ni(CN)₄·G (M = Co, Ni or Cd; G = Benzene, Naphthalene, Anthracene, Phenanthrene or Biphenyl)". Journal of Inclusion Phenomena and Macrocyclic Chemistry. 39 (1/2): 175–180. doi:10.1023/A:1008141726024. S2CID 91455271.
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^ Niu, T.; Crisci, G.; Lu, J.; Jacobson, A. J. (15 May 1998). "Diaquacobalt Tetracyanonickelate Tetrahydrate". Acta Crystallographica Section C. 54 (5): 565–567. Bibcode:1998AcCrC..54..565N. doi:10.1107/S0108270197018003.
^ ^a ^b Ham, William K.; Weakley, Timothy J.R.; Page, Catherine J. (November 1993). "Synthesis and Crystal Structure of Cd(H₂O)₂Ni(CN)₄·4H₂O". Journal of Solid State Chemistry. 107 (1): 101–107. Bibcode:1993JSSCh.107..101H. doi:10.1006/jssc.1993.1327.
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