Mercury(II) cyanide


Mercury 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.

Molecular and crystal structure

At ambient temperature and ambient pressure, Hg2 takes the form of tetragonal crystals. These crystals are composed of nearly linear Hg2 molecules with a C-Hg-C bond angle of 175.0° and an Hg-C-N bond angle of 177.0°. Raman spectra show that the molecules distort at higher pressures. Between 16-20 kbar, the structure undergoes a phase transition as the Hg 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 Cd2. 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.
As in the solid state, aqueous solution, Hg2 molecules are linear.

Synthesis

Mercuric cyanide can be prepared by mixing yellow mercury oxide with hydrocyanic acid in the following chemical reaction which is generally carried out by passing HCN gas into HgO in water. When soluble Hg2 is formed, the solution is evaporated to crystallize the product.
Hg2 can also be prepared by mixing HgO, finely powdered Prussian blue. In addition, it can be produced by reacting mercuric sulfate with potassium ferrocyanide in water:
Another method to generate mercuric cyanide is through the disproportionation of mercury derivatives. In these reactions, metallic mercury precipitates, and Hg2 remains in solution:

Reactions

It rapidly decomposes in acid to give off hydrogen cyanide. It is photosensitive, becoming darker in color.
Mercury cyanide catalyzes the Koenigs–Knorr reaction for the synthesis of glycosides. Cyanogen, 2, forms upon heating dry mercury cyanide, but the method is inferior to other routes:
Coordination polymers can be synthesized from Hg2 building blocks. Large single crystals of 2] form upon treating CuCl2, the soft Lewis acid Hg2, and N,N,N',N'-tetramethylethylenediamine. The migration of two labile chloride ligands from harder Cu to softer Hg drives the formation of the crystal.

Past applications

Mercuric cyanide was once used as an antiseptic, but this practice has been discontinued due to its toxicity. 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. Hg2 is also used in photography. It is still used in homeopathy under the Latin name Hydrargyrum bicyanatum.

Toxicology

Mercury cyanide is poison with health hazard classification 3, having toxicity LD50 Mouse oral 33 mg/kg and Dog sc 2710 ug/kg. Its high toxicity is due to mercury and its two cyanide groups.
Because of the high solubility of mercury 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.