Thiocyanate

Thiocyanate (also known as sulphocyanate or thiocyanide or rhodanide) 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] ⁻, where in 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.

tructure, bonding and coordination chemistry

Thiocyanate shares its negative charge approximately equally between sulfur and nitrogen. Consequently, 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)] Cl₂Greenwood&Earnshaw p. 326] .

Organic thiocyanates

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 [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] . In isothiocyanates, the substituent is attached to nitrogen: R−N=C=S has a S-C double bond and a C-N double bondOrganic 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 presence of the [Fe(NCS)(H₂O)₅] ²⁺.

References