Thiocyanate

Thiocyanate (also known as rhodanide) is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates. Mercury(II) thiocyanate was formerly used in pyrotechnics.

Thiocyanate is analogous to the cyanate ion, [OCN]−, wherein oxygen is replaced by sulfur. [SCN]− is one of the pseudohalides, due to the similarity of its reactions to that of halide ions. Thiocyanate used to be known as rhodanide (from a Greek word for rose) because of the red colour of its complexes with iron. Thiocyanate is produced by the reaction of elemental sulfur or thiosulfate with cyanide:
 * 8 CN− +  S8  →  8 SCN−
 * CN− +  S2O32−  →  SCN−  +  SO32−

The second reaction is catalyzed by the enzyme sulfotransferase known as rhodanase and may be relevant to detoxification of cyanide in the body.

Structure, bonding and coordination chemistry
Thiocyanate shares its negative charge approximately equally between sulfur and nitrogen. As a consequence, thiocyanate can act as a nucleophile at either sulfur or nitrogen — it is an ambidentate ligand. [SCN]− can also bridge two (M−SCN−M) or even three metals (>SCN− or −SCN<). Experimental evidence leads to the general conclusion that class A metals (hard acids) tend to form N-bonded thiocyanate complexes, whereas class B metals (soft acids) tend to form S-bonded thiocyanate complexes. Other factors, e.g. kinetics and solubility, are sometimes involved, and linkage isomerism can occur, for example [Co(NH3)5(NCS)]Cl2 and [Co(NH3)5(SCN)]Cl2.

Organic thiocyanates
Organic and transition metal derivatives of the thiocyanate ion can exist as "linkage isomers." In thiocyanates, the organic group (or metal ion) is attached to sulfur: R−S−C≡N has a S-C single bond and a C-N triple bond. In isothiocyanates, the substituent is attached to nitrogen: R−N=C=S has a S-C double bond and a C-N double bond: Organic thiocyanates are hydrolyzed to thiocarbamates in the Riemschneider thiocarbamate synthesis.

Test for iron(III)
If [SCN]− is added to a solution containing iron (III) ions (Fe3+), a blood red solution is formed due to the formation of [Fe(NCS)(H2O)5]2+.

Biological chemistry of thiocyanate in medicine
Thiocyanate is known to be an important part in the biosynthesis of hypothiocyanite by a lactoperoxidase. Thus the complete absence of thiocyanate or reducted thiocyanate in the human body, (e.g., cystic fibrosis) is of high importance in the human host defense system.

Thiocyanate is a metabolite of sodium nitroprusside, after rhodanese catalyses its reaction with thiosulfate.