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. In their crystals, the tetracyanonickelate(II) anions are often arranged in a columnar structure (e.g. in K2[Ni(CN)4] ICSD number 24099.). 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−. Tetracyanonickelate(III) can add two more cyanide groups to form hexacyanonickelate(III).
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.
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.
formulanameNi OxstructureRemarksreferencesNa2[Ni(CN)4]·3H2Odisodium tetracyanonickelate(II)2triclinic 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=262.81orange; Ni(CN)4 planes parallelK2[Ni(CN)4]·H2Opotassium tetracyanonickelate(II)2monoclinic orange-red; dehydrate at 110°K2[Ni(CN)4]Potassium tetracyanonickelate2monoclinic a=4.294 Å, b=7.680 Å, c=13.02 Å, β=87°16orange-yellow; CN forms a square around NiCa[Ni(CN)4]·5H2Ocalcium tetracyanonickelate2orthorhombic Pcab a=18.18 Å, b=18.86 Å, c=6.774 Å, Z=8, V=2195 Å3, density=1.774CN forms distorted square around Ni; yellow when light polarized in elongation direction, colourless ⊥Co·2H2O[Ni(CN)4]·4H2Odiaquacobalt tetracyanonickelate tetrahydrate2orthorhombic a=12.178 Å, b=13.885 Å, c=7.143 Å, V=1207.8 Z=4orange MW=329.82Ni(NH3)2[Ni(CN)4]·C6H6Hofmann clathrate2/2benzene 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.Rb2[Ni(CN)4]·H2Orubidium tetracyanonickelate(II)2triclinic P a=8.602 Å, b=9.693 Å, c=12.006 Å, α = 92.621°, β= 94.263°, γ =111.79° V=924.0 Å3orange needles; Ni(CN)4 planes parallel; water in 5% excessSr[Ni(CN)4]·5H2Ostrontium tetracyanonickelate(II)2monoclinic C2/m, a=10.356 Å, b=15.272 Å, c=7.133 Å, α=98.55°, V=1115.6 Å3orange; Ni(CN)4 planes parallelCd·2H2O[Ni(CN)4]·4H2Odiaquacadmium tetracyanonickelate tetrahydrate2orthorhombic Pnma, a = 12.393 Å, b = 14.278 Å, c = 7.427 Å, Z = 4, V=1314 Å3, density=1.937MW=383.27Cs2[Ni(CN)4]·1.05H2Ocesium tetracyanonickelate(II)2hexagonal, P61, a = 9.526 Å, c = 19.043 Å, V = 1496.5 Å3 Z=6golden yellow; Ni(CN)4 planes arranged in a spiralCsKNi(CN)4cesium potassium tetracyanonickelate2triclinic 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=2.55orangeBa[Ni(CN)4]·4H2Obarium tetracyanonickelate(II)2monoclinicred(Dimethylformamide)4EuNi(CN)4europium(II) tetracyanonickelate2triclinic P1̄, a=8.902Å, b=10.947Å, c = 12.464Å, α = 82.99°, β = 86.86°, γ = 84.92°, Z = 2Tl2[Ni(CN)4]·1.05H2Othallium(I) tetracyanonickelate2monoclinic a=6.154 b=7.282 c=9.396 β=104.29 V=408.0 Z=2 density=4.652bright yellow orange; chains of TlNi(UO2)2(dmso)4(OH)2[Ni(CN)4]2monoclinic C2/c a=21.522 Å, b=10.2531 Å, c=13.3170 Å, β=111.943° V=2725.8 ÅyellowK4[(CN)3Ni-Ni(CN)3]potassium hexacyanodinickelate(I)1"Bellucci's salt"
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