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.[2]

Mercury(II) cyanide
Names
IUPAC name
dicyanomercury
Other names
mercuric cyanide; cyanomercury; mercury cyanide; mercury dicyanide; hydrargyri cyanidum[1]
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.008.857
UNII
Properties
Hg(CN)2
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)[2] (decomposes)
9.3 g/100 mL (14 °C)
53.9 g/100 mL (100 °C)[3]
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
1.645
Hazards
T+ N
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 4: Very short exposure could cause death or major residual injury. E.g. VX gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazards (white): no code
0
4
2
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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Infobox references

Molecular and crystal structure

At ambient temperature and ambient pressure, Hg(CN)2 takes the form of tetragonal crystals.[2] 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[3] 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.[5]

As in the solid state, aqueous solution, Hg(CN)2 molecules are linear.[3]

Synthesis

Mercuric cyanide can be prepared by mixing yellow mercury oxide with hydrocyanic acid in the following chemical reaction[3] 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, finely powdered Prussian blue.[3][6] In addition, it can be produced by reacting mercuric sulfate with potassium ferrocyanide in water:[6]

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:[6]

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

Reactions

It rapidly decomposes in acid to give off hydrogen cyanide. It is photosensitive, becoming darker in color.[7]

Mercury cyanide catalyzes the Koenigs–Knorr reaction for the synthesis of glycosides.[2] Cyanogen, (CN)2, forms upon heating dry mercury cyanide, but the method is inferior to other routes:[8]

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.[9]

Past applications

Mercuric cyanide was once used as an antiseptic, but this practice has been discontinued due to its toxicity.[10] One example of this was the treatment of syphilis: a solution of 5-10 grains in an ounce of water, painted on with a camel-hair brush, was applied to syphilitic sores of the tongue or mouth. Hg(CN)2 is also used in photography.[11] It is still used in homeopathy under the Latin name Hydrargyrum bicyanatum.

Toxicology

Mercury(II) cyanide is poison with health hazard classification 3, having toxicity LD50 Mouse oral 33 mg/kg and Dog sc 2710 ug/kg.[12] Its high toxicity is due to mercury and its two cyanide groups. Because of the high solubility of mercury(II) cyanide in water, it can absorb from skin or inhalation and cause death. Upon absorption it is readily metabolized into both cyanide and mercury, thereby resulting in the symptoms of poisoning by both agents. It is highly dangerous for the environment.

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References
  1. "Hydrargyrum. Mercury. Part 5." http://chestofbooks.com/health/materia-medica-drugs/Manual-Pharmacology/Hydrargyrum-Mercury-Part-5.html (accessed April 1, 2009).
  2. 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. http://www.mrw.interscience.wiley.com/eros/articles/rm034/sect0-fs.html
  3. 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.
  4. http://cameochemicals.noaa.gov/chemical/3829
  5. Wong, P.T.T. J. Chem. Phys. 1984, 80(12), 5937-41.
  6. Miller, W.L. Elements of Chemistry: Organic chemistry, 5th ed. New York: John Wiley & Sons, 1880, p. 100.
  7. Brunton, L.T. A Text-Book Of Pharmacology, Therapeutics And Materia Medica. London: MacMillan & Co., 1885.
  8. Brotherton, T.K.; Lynn, J.W. Chemical Reviews 1959, 59(5), 841-883, 844-846.
  9. Draper, N.D. Chemistry of Materials 2003, 15(8), 1612–1616.
  10. Benaissa, M.L.; Hantson, P.; Bismuth, C.; Baud, F.J. Intensive Care Med. 1995, 21(12), 1051-1053.
  11. "Cyanides, Cyanide Oxides and Complex Cyanides." http://www.dncustoms.gov.vn/web_eglish/bieu_thue/E_HTM/E2837.HTM (accessed April 30, 2009).
  12. Pubchem. "Mercuric cyanide". pubchem.ncbi.nlm.nih.gov. Retrieved 2018-03-22.
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