2_Xray.jpg) | |
 | |
| Names | |
|---|---|
| Other names Mercuric thiocyanate Mercuric sulfocyanate | |
| Identifiers | |
| |
3D model (JSmol) | |
| ChemSpider | |
| ECHA InfoCard | 100.008.886 |
| EC Number |
|
| UNII | |
| UN number | 1646 |
| |
| |
| Properties | |
| Hg(SCN)2 | |
| Molar mass | 316.755 g/mol |
| Appearance | Whitemonoclinic powder |
| Odor | odorless |
| Density | 3.71 g/cm3, solid |
| Melting point | 165 °C (329 °F; 438 K) (decomposes) |
| 0.069 g/100 mL | |
| Solubility | Soluble in dilutehydrochloric acid,KCN,ammonia slightly soluble inalcohol,ether |
| −96.5·10−6 cm3/mol | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | highly toxic |
| GHS labelling:[1] | |
   | |
| Danger | |
| H300,H310,H330,H373,H410 | |
| P260,P262,P270,P271,P273,P280,P284,P301+P316,P302+P352,P304+P340,P316,P319,P320,P321,P330,P361,P364,P391,P403+P233,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 46 mg/kg (rat, oral) |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Mercury(II) thiocyanate (Hg(SCN)2) is an inorganicchemical compound, thecoordination complex ofHg2+ and thethiocyanateanion. It is a white powder. It will produce a large, winding "snake" when ignited, an effect known as thePharaoh's serpent.[2]
The first synthesis of mercury thiocyanate was probably completed in 1821 byJöns Jacob Berzelius:[citation needed]
Evidence for the first pure sample was presented in 1866 prepared by a chemist named Otto Hermes.[2] It is prepared by treating solutions containing mercury(II) ions with a thiocyanate ion source. The lowsolubility product of mercury thiocyanate causes it to precipitate from the solution.[3] Most syntheses are achieved by precipitation:
The compound adopts a polymeric structure withHg2+ centres linearly coordinated to two S atoms with a distance of2.381 Å. Four weakHg2+-N interactions are indicated with distances of2.81 Å.[4][better source needed]
Mercury thiocyanate has a few uses in chemical synthesis.
It is the precursor to otherthiocyanate complexes such as potassium tris(thiocyanato)mercurate(II) (K[Hg(SCN)3]) and caesium tris(thiocyanato)mercurate(II) (Cs[Hg(SCN)3]). TheHg(SCN)3− ion can also exist independently and is easily generated from the compounds above, amongst others.[5]
Organic halides attackHg(SCN)2 to give a mercuric halide and a mixture of the correspondingthiocyanate andisothiocyanate.[6]
Mercuric thiocyanate catalyzes HSCN or BrSCN addition (either reagent formedin situ) toalkynes.[7]
Mercury thiocyanate improves detection limits of chloride ions in water byUV-visible spectroscopy. The method involves the addition of mercury thiocyanate to a solution with an unknown concentration of chloride ions and iron as areagent. The chloride ions cause the mercury thiocyanate salt to dissociate and the thiocyanate ion to bind Fe(III), which absorbs intensely at450 nm. This absorption allows for the measurement of the concentration of the iron complex. This value allows one to calculate the concentration of chloride. This technique was a standard method for the determination of chloride ions in laboratories worldwide.[when?][8]
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 andgum 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.[9]
When heated, mercury(II) thiocyanate decomposes in anexothermic reaction that produces a large mass of coiling, serpent-like solids.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.[incomprehensible] 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 producedβ-HgS (black mercury sulfide) and vaporization of the resulting mercury from the outermost and hottest layers of the solid.[10]
The decomposition ofHg(SCN)2 is exothermic on its own, and theCS2 produced ignites easily and burns off. TheC3N4 product is a simplification; the actual product contains 0.5% hydrogen and is likely to consist of sheets oftriazine rings linked by−N= and−NH− groups similar tog−C3N4 and was found to contain nano-particles ofβ-HgS (black mercury sulfide).[10]
The number of resonance structures ofheptazine 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 toLiebig'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.[10] The generalized reaction is as follows:[citation needed]
