Mercury(II) thiocyanate. Mercury(II) thiocyanate () is an inorganic chemical compound, the coordination complex of  and the thiocyanate anion. It is a white powder.  It will produce a large, winding "snake" when ignited, an effect known as the Pharaoh's serpent.

Synthesis and structure
The first synthesis of mercury thiocyanate was probably completed in 1821 by Jöns Jacob Berzelius:

Evidence for the first pure sample was presented in 1866 prepared by a chemist named Otto Hermes. It is prepared by treating solutions containing mercury(II) and thiocyanate ions. The low solubility product of mercury thiocyanate causes it to precipitate from the solution. Most syntheses are achieved by precipitation:

The compound adopts a polymeric structure with Hg2+ centres linearly coordinated to two S atoms with a distance of 2.381 Å.  Four weak Hg2+--N interactions are indicated with distances of 2.81 Å.

Reactions
Mercury thiocyanate has a few uses in chemical synthesis. 

It is the precursor to other thiocyanate complexes such as potassium tris(thiocyanato)mercurate(II) () and caesium tris(thiocyanato)mercurate(II) (). The  ion can also exist independently and is easily generated from the compounds above, amongst others.

Organic halides attack Hg(SCN)2 to give a mercuric halide and a mixture of the corresponding thiocyanate and isothiocyanate.  

Mercuric thiocyanate catalyzes HSCN or BrSCN addition (either reagent formed in situ) to alkynes.

Use in chloride analysis
Mercury thiocyanate improve detection limits in the determination of chloride ions in water by UV-visible spectroscopy. This technique was a standard method for the determination of chloride ions in laboratories worldwide. The method involves the addition of mercury thiocyanate to a solution with an unknown concentration of chloride ions and iron as a reagent. The chloride ions cause the mercury thiocyanate salt to dissociate and the thiocyanate ion to bind Fe(III), which absorbs intensely at 450 nm. This absorption allows for the measurement of the concentration of the iron complex. This value allows one to calculate the concentration of chloride.

Pharaoh's serpent

Mercury thiocyanate was formerly used in pyrotechnics causing an effect known as the Pharaoh's serpent or Pharaoh's snake. The fireworks versions were a mixture with a small amount of potassium nitrate and gum arabic as a binder. This use ceased in most countries in the early 20th century due to the toxicity of mercury, and the existence of a superior alternatives.

Chemistry of the reaction
When heated, mercury(II) thiocyanate decomposes in an exothermic reaction that can produce a large mass of coiling, serpent-like solid.  An inconspicuous flame, which is usually the blue color of burning carbon disulfide but which can be yellow from impurities or incidental combustion of flammable materials on the surface it is ignited on. The resulting solid can range from dark graphite gray to light tan in colour with the inside generally much darker than the outside.   This was found to be due to decomposition of the produced β-HgS (black mercury sulfide) and vaporization of the resulting mercury from the othermost and hottest layers of the solid.

The decomposition of  is exothermic on its own, and the  produced ignites easily and burns off.  The  product is a simplification; the actual product contains 0.5% hydrogen and is likely to consist of sheets of triazine rings linked by  and  groups similar to  and was found to contain nano-particles of β-HgS (black mercury sulfide).

The number of resonance structures of heptazine and triazine, varying molecular weights of samples, and the fluorescense of the product made acquiring spectra difficult even by relatively exotic methods of NMR (with one spectrum acquisition being run for 12 days straight to get a mostly clean reading).  Because of this, a  heptazine-based structure similar to Liebig's melon, a compound initially prepared around the same time that the pharoah's snake reaction was discovered, was not ruled out by the authors as a partial component of the solid material.  The generalized reaction is as follows:

 
  (not all mercury sulfide decomposes)

 is not a product of this decomposition.  Cyanogen is generally only produced when  or similar is heated to decomposition, and early attempts to form  via the same route starting at this compound failed and only generated ,  and .

References

External links
"Pharaoh's snake". YouTube. September 2, 2008.

Thiocyanates
Mercury(II) compounds