Cyanonickelate Explained

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.

formula	name	Ni Ox	structure	Remarks	references
Na2[Ni(CN)<sub>4</sub>]·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 Å^3 FW=262.81	orange; Ni(CN)4 planes parallel	^[5]
K2[Ni(CN)<sub>4</sub>]·H2O	potassium tetracyanonickelate(II)	2	monoclinic	orange-red; dehydrate at 110°
K2[Ni(CN)<sub>4</sub>]	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
Ca[Ni(CN)<sub>4</sub>]·5H2O	calcium tetracyanonickelate	2	orthorhombic Pcab a=18.18 Å, b=18.86 Å, c=6.774 Å, Z=8, V=2195 Å^3, density=1.774	CN forms distorted square around Ni; yellow when light polarized in elongation direction, colourless ⊥	^[6]
Co·2H2O[Ni(CN)<sub>4</sub>]·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	^[7]
Ni(NH3)2[Ni(CN)<sub>4</sub>]·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.
Rb2[Ni(CN)<sub>4</sub>]·H2O	rubidium tetracyanonickelate(II)	2	triclinic 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 Å^3	orange needles; Ni(CN)4 planes parallel; water in 5% excess	^[8]
Sr[Ni(CN)<sub>4</sub>]·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 Å^3	orange; Ni(CN)4 planes parallel
Cd·2H2O[Ni(CN)<sub>4</sub>]·4H2O	diaquacadmium tetracyanonickelate tetrahydrate	2	orthorhombic Pnma, a = 12.393 Å, b = 14.278 Å, c = 7.427 Å, Z = 4, V=1314 Å^3, density=1.937	MW=383.27	^[9]
Cs2[Ni(CN)<sub>4</sub>]·1.05H2O	cesium tetracyanonickelate(II)	2	hexagonal, P61, a = 9.526 Å, c = 19.043 Å, V = 1496.5 Å^3 Z=6	golden yellow; Ni(CN)4 planes arranged in a spiral	^[10]
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
Ba[Ni(CN)<sub>4</sub>]·4H2O	barium tetracyanonickelate(II)	2	monoclinic	red	^[11]
(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		^[12]
Tl2[Ni(CN)<sub>4</sub>]·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	^[13]
(UO2)2(dmso)4(OH)2[Ni(CN)<sub>4</sub>]		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)<sub>3</sub>Ni-Ni(CN)<sub>3</sub>]	potassium hexacyanodinickelate(I)	1			"Bellucci's salt"^[15]

Notes and References

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2. Vannerberg. Nils Gosta. The Crystal Structure of K₂Ni(CN)₄. Acta Chemica Scandinavica. 1964. 18. 10. 2385–2391. 29 April 2016. 10.3891/acta.chem.scand.18-2385. free. ICSD number 24099.
3. Wang. Yi Lai. Beach. Mark W.. Pappenhagen. Thomas L.. Margerum. Dale W.. Mixed-ligand complexes of tetracyanonickelate(III) and dynamic Jahn-Teller distortions of hexacyanonickelate(III). Inorganic Chemistry. November 1988. 27. 24. 4464–4472. 10.1021/ic00297a025.
4. Şenyel. Mustafa. Raci Sertbakan. T.. Kürkçüoğ. Güneş. Kasap. Ergün. Kantarci. Ziya. 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. 2001. 39. 1/2. 175–180. 10.1023/A:1008141726024. 91455271.
5. Ptasiewicz-Bak. H.. Olovsson. I.. McIntyre. G. J.. Structure, Charge and Spin Density in Na₂Ni(CN)₄·3H₂O at 295 and 30 K. Acta Crystallographica Section B. 1 October 1998. 54. 5. 600–612. 10.1107/S0108768198002286. 1998AcCrB..54..600P .
6. Holt. E. M.. Watson. K. J.. The Crystal Structure of Calcium Tetracyanonickelate (II).. Acta Chemica Scandinavica. 1969. 23. 1. 14–28. 29 April 2016. 10.3891/acta.chem.scand.23-0014. free.
7. Niu. T.. Crisci. G.. Lu. J.. Jacobson. A. J.. Diaquacobalt Tetracyanonickelate Tetrahydrate. Acta Crystallographica Section C. 15 May 1998. 54. 5. 565–567. 10.1107/S0108270197018003. 1998AcCrC..54..565N .
8. Fronczek. Frank R.. Delord. Terry J.. Watkins. Steven F.. Gueorguieva. Petia. Stanley. George G.. Zizza. Annegret S.. Cornelius. Jeffrey B.. Mantz. Yves A.. Musselman. Ronald L.. A Solid-State Spectral Effect in Eclipsed Tetracyanonickelates: X-ray Crystal Structure, Polarized Specular Reflectance Spectroscopy, and ZINDO Modeling of Sr[Ni(CN)₄]·5H₂O, Rb₂[Ni(CN)₄]·H₂O, and Na₂[Ni(CN)₄]·3H₂O. Inorganic Chemistry. November 2003. 42. 22. 7026–7036. 10.1021/ic0345222. 14577768.
9. Ham. William K.. Weakley. Timothy J.R.. Page. Catherine J.. Synthesis and Crystal Structure of Cd(H₂O)₂Ni(CN)₄·4H₂O. Journal of Solid State Chemistry. November 1993. 107. 1. 101–107. 10.1006/jssc.1993.1327. 1993JSSCh.107..101H.
10. Cornelius. Jeffrey B.. Trapp. Robert M.. Delord. Terry J.. Fronczek. Frank R.. Watkins. Steven F.. Orosz. Jill Jasin. Musselman. Ronald L.. One-Dimensional Collective Electronic Effects in the Helically Stacked Cs₂[Ni(CN)₄]·H₂O and Cs₂[Pt(CN)₄]·H₂O: X-ray Structure, Polarized Specular Reflectance, and ZINDO Calculations. Inorganic Chemistry. May 2003. 42. 9. 3026–3035. 10.1021/ic026101a. 12716197.
11. McCullough. R.L.. Jones. L.H.. Crosby. G.A.. An analysis of the vibrational spectrum of the tetracyanonickelate(II) ion in a crystal lattice. Spectrochimica Acta. January 1960. 16. 8. 929–944. 10.1016/0371-1951(60)80057-4. 1960AcSpe..16..929M.
12. Knoeppel. David W.. Shore. Sheldon G.. Unusual One-Dimensional Ladder Structures Containing Divalent Europium and the Tetracyanometalates Ni(CN)₄ and Pt(CN)₄. Inorganic Chemistry. January 1996. 35. 18. 5328–5334. 10.1021/ic960337v.
13. Maliarik. Mikhail. Nagle. Jeffrey K.. Ilyukhin. Andrey. Murashova. Elena. Mink. János. Skripkin. Mikhail. Glaser. Julius. Kovacs. Margit. Horváth. Attila. Metal−Metal Bonding in Tetracyanometalates (M = Pt, Pd, Ni) of Monovalent Thallium. Crystallographic and Spectroscopic Characterization of the New Compounds Tl₂Ni(CN)₄ and Tl₂Pd(CN)₄. Inorganic Chemistry. May 2007. 46. 11. 4642–4653. 10.1021/ic062092k. 17474737.
14. Maynard. Branson A.. Lynn. K. Sabrina. Sykora. Richard E.. Gorden. Anne E. V.. Emission, Raman Spectroscopy, and Structural Characterization of Actinide Tetracyanometallates. Inorganic Chemistry. 6 May 2013. 52. 9. 4880–4889. 10.1021/ic302459z. 23594182. 0020-1669.
15. 10.1002/anie.197000711. Structure of the Anion in Solid K4[Ni2(CN)6]. 1970. Jarchow. O.. Schulz. H.. Nast. R.. Angewandte Chemie International Edition in English. 9. 71.