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The structure and bonding of the thiocyanate ion

Thiocyanate (also known as sulphocyanate or thiocyanide) is the anion, [SCN]. Common compounds 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 was formerly 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 latter reaction is catalyzed by the enzyme sulfotransferase known as rhodanase and may be relevant to detoxification of cyanide in the body.

Linkage isomerism

Organic and transition metal derivatives of the thiocyanate ion can exist as "linkage isomers." In thiocyanates, the organic group or metal 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

Phenylthiocyanate and phenylisothiocyanate are linkage isomers and are bonded differently

Test for iron(III)

The blood-red coloured complex pentaaqua(thiocyanato-N)iron(III), [Fe(NCS)(H2O)5]2+, indicates the presence of Fe3+ in solution

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

General reading

  • Guy, R. G. "Syntheses and Preparative Applications of Thiocyanates" in "Chemistry of Cyanates and Their Derivatives," vol II. Patai, S., (Editor), John Wiley, 1977. New York