Cyanonickelate

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Four cyano groups are arranged in a square in the tetracyanonickelate ion.
Potassium tetracyanonickelate(II) solution.
Potassium tetracyanonickelate(II) solid.

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 D4h. 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
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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