Mercury(II) cyanide

Mercury(II) cyanide, also known as mercuric cyanide, is a poisonous compound of mercury and cyanide. 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, Hg(CN)2 takes the form of tetragonal crystals. 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 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.

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

Synthesis
Mercuric cyanide is formed from aqueous hydrogen cyanide and mercuric oxide:
 * HgO + 2 HCN  ->  Hg(CN)2  +  H2O

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

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

Past applications
The use of mercuric cyanide as an antiseptic was discontinued due to its toxicity. Hg(CN)2 is also used in photography.

Toxicology
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.