Mercury(II) cyanide

At ambient temperature and ambient pressure, Hg(CN)2 takes the form of tetragonal crystals. 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: : 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: : 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. : Hg(CN)2 → (CN)2 + Hg It serves as a source of Hg2+ in its reaction with sodium tetracarbonylcobalt: : 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 4, having an oral LD50 of 33 milligrams per kilogram in mice and a subcutaneous LD50 of 2.7 milligrams per kilogram in dogs. ==References==