Mercury(II) cyanide

Mercury(II) cyanide, also known as mercuric cyanide, is a compound of mercury. It is an odorless, toxic white powder. It is highly soluble in polar solvents such as water, alcohol, and ammonia; slightly soluble in ether; and insoluble in benzene and other hydrophobic solvents.[3]

Mercury(II) cyanide
Mercuric cyanide.png
IUPAC name
Other names
mercuric cyanide; cyanomercury; mercury cyanide; mercury dicyanide; hydrargyri cyanidum[1]
3D model (JSmol)
ECHA InfoCard 100.008.857 Edit this at Wikidata
EC Number
  • 209-741-6
UN number 1636
  • InChI=1S/2CN.Hg/c2*1-2;
  • InChI=1/2CN.Hg/c2*1-2;/rC2HgN2/c4-1-3-2-5
  • C(#N)[Hg]C#N
Molar mass 252.63 g/mol
Appearance colorless crystals or white powder
Odor odorless
Density 3.996 g/cm3
Melting point 320 °C (608 °F; 593 K)[3] (decomposes)
9.3 g/100 mL (14 °C)
53.9 g/100 mL (100 °C)[2]
Solubility 25 g/100 mL (methanol, 19.5 °C)
soluble in ethanol, ammonia, glycerin
slightly soluble in ether
insoluble in benzene
−67.0·10−6 cm3/mol
GHS labelling:[4]
GHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
H300, H310, H330, H373, H410
P260, P262, P264, P270, P271, P273, P280, P284, P301+P310, P302+P350, P304+P340, P310, P314, P320, P322, P330, P361, P363, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
26 mg/kg
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Molecular and crystal structureEdit

At ambient temperature and ambient pressure, Hg(CN)2 takes the form of tetragonal crystals.[3] These crystals are composed of nearly linear Hg(CN)2 molecules with a C-Hg-C bond angle of 175.0° and an Hg-C-N bond angle of 177.0° (Aylett[2] gives slightly different values of 189° and 175°, respectively). Raman spectra show that the molecules distort at higher pressures. Between 16-20 kbar, the structure undergoes a phase transition as the Hg(II) center changes from 2- to 4-coordinate as the CN groups bind to neighboring Hg centers forming via Hg-N bonds. The coordination geometry thus changes from tetragonal to tetrahedral, forming a cubic crystal structure, analogous to the structure of Cd(CN)2. Due to the ambidentate nature of the CN ligands, this tetrahedral structure is distorted, but the distortion lessens with increasing pressure until the structure becomes nearly perfectly tetrahedral at >40 kbar.[6]

As in the solid state, in aqueous solution, Hg(CN)2 molecules are linear.[2]


Mercuric cyanide can be prepared by mixing yellow mercury oxide with hydrocyanic acid in the following chemical reaction[2] which is generally carried out by passing HCN gas into HgO in water. When soluble Hg(CN)2 is formed, the solution is evaporated to crystallize the product.[1]

HgO + 2 HCN → Hg(CN)2 + H2O

Hg(CN)2 can also be prepared by mixing HgO with finely powdered Prussian blue.[2][7] In addition, it can be produced by reacting mercuric sulfate with potassium ferrocyanide in water:[7]

K4Fe(CN)6 + 3 HgSO4 → 3 Hg(CN)2 + 2 K2SO4 + FeSO4

Another method to generate mercuric cyanide is through the disproportionation of mercury(I) derivatives. In these reactions, metallic mercury precipitates, and Hg(CN)2 remains in solution:[7]

Hg2(NO3)2 + 2 KCN → Hg + Hg(CN)2 + 2 KNO3


It rapidly decomposes in acid to give off hydrogen cyanide. It is photosensitive, becoming darker in color.[8]

Mercury cyanide catalyzes the Koenigs–Knorr reaction for the synthesis of glycosides.[3] Cyanogen, (CN)2, forms upon heating dry mercury cyanide, but the method is inferior to other routes:[9]

Hg(CN)2 → (CN)2 + Hg

Coordination polymers can be synthesized from Hg(CN)2 building blocks. Large single crystals of [(tmeda)Cu-[Hg(CN)2]2][HgCl4] form upon treating CuCl2, the soft Lewis acid Hg(CN)2, and N,N,N',N'-tetramethylethylenediamine (TMEDA). The migration of two labile chloride ligands from harder Cu(II) to softer Hg(II) drives the formation of the crystal.[10]

Past applicationsEdit

The use of mercuric cyanide as an antiseptic was discontinued due to its toxicity.[11] Hg(CN)2 is also used in photography.[12] It is still used in homeopathy under the Latin name Hydrargyrum bicyanatum.


Mercury(II) cyanide is poison with health hazard classification 3, having an oral LD50 of 33 milligrams per kilogram in mice and a subcutaneous LD50 of 2.7 milligrams per kilogram in dogs.[13]


  1. ^ a b "Hydrargyrum. Mercury. Part 5." (accessed April 1, 2009).
  2. ^ a b c d e Aylett, B.J. "Mercury (II) Pseudohalides: Cyanide, Thiocyanate, Selenocyanate, Azide, Fulminate." Comprehensive Inorganic Chemistry 3:304-306. J.C. Bailar, Harry Julius Emeléus, Sir Ronald Nyholm, and A.F. Trotman-Dickenson, ed. Oxford: Pergamon Press, 1973; distributed by Compendium Publishers (Elmsford, NY), p. 304.
  3. ^ a b c d Kocovsky, P., G. Wang, and V. Sharma. "Mercury(II) Cyanide." e-EROS Encyclopedia of Reagents for Organic Synthesis. Chichester, UK: John Wiley & Sons, Ltd., 2001.
  4. ^ "Mercuric cyanide". Retrieved 23 December 2021.
  5. ^ "MERCURIC CYANIDE | CAMEO Chemicals | NOAA".
  6. ^ Wong, P.T.T. J. Chem. Phys. 1984, 80(12), 5937-41.
  7. ^ a b c Miller, W.L. Elements of Chemistry: Organic chemistry, 5th ed. New York: John Wiley & Sons, 1880, p. 100.
  8. ^ Brunton, L.T. A Text-Book Of Pharmacology, Therapeutics And Materia Medica. London: MacMillan & Co., 1885.
  9. ^ Brotherton, T.K.; Lynn, J.W. Chemical Reviews 1959, 59(5), 841-883, 844-846.
  10. ^ Draper, Neil D.; Batchelor, Raymond J.; Sih, Bryan C.; Ye, Zuo-Guang; Leznoff, Daniel B. (2003). "Synthesis, Structure, and Properties of [(tmeda)Cu[Hg(CN)2]2][HgCl4]: A Non-Centrosymmetric 2-D Layered System that Shows Strong Optical Anisotropy". Chemistry of Materials. 15 (8): 1612–1616. doi:10.1021/cm021716r.
  11. ^ Benaissa, M.L.; Hantson, P.; Bismuth, C.; Baud, F.J. Intensive Care Med. 1995, 21(12), 1051-1053.
  12. ^ "Cyanides, Cyanide Oxides and Complex Cyanides." (accessed April 30, 2009).
  13. ^ Pubchem. "Mercuric cyanide". Retrieved 2018-03-22.

External linksEdit