Thiocyanate is analogous to thecyanate ion,[OCN]−, whereinoxygen is replaced bysulfur.[SCN]− is one of thepseudohalides, due to the similarity of its reactions to that ofhalide ions. Thiocyanate used to be known as rhodanide (from aGreek word forrose) because of the red colour of its complexes withiron.
Thiocyanate is produced by the reaction of elemental sulfur orthiosulfate withcyanide:
8 CN− + S8 → 8 SCN−
CN− + S2O2−3 → SCN− + SO2−3
The second reaction is catalyzed bythiosulfate sulfurtransferase, a hepatic mitochondrial enzyme, and by other sulfur transferases, which together are responsible for around 80% of cyanide metabolism in the body.[2]
Thiocyanate[6] is known to be an important part in the biosynthesis ofhypothiocyanite by alactoperoxidase.[7][8][9] Thus the complete absence of thiocyanate or reduced thiocyanate[10] in the human body, (e.g.,cystic fibrosis) is damaging to the human host defense system.[11][12]
Thiocyanate is a potent competitive inhibitor of the thyroidsodium-iodide symporter.[13] Iodine is an essential component ofthyroxine. Since thiocyanates will decrease iodide transport into the thyroid follicular cell, they will decrease the amount of thyroxine produced by the thyroid gland. As such, foodstuffs containing thiocyanate are best avoided by iodide deficient hypothyroid patients.[14]
In the early 20th century, thiocyanate was used in the treatment ofhypertension, but it is no longer used because of associated toxicity.[15]Sodium nitroprusside, a metabolite of which is thiocyanate, is however still used for the treatment of ahypertensive emergency.Rhodanese catalyzes the reaction of sodium nitroprusside (like othercyanides) withthiosulfate to form the metabolite thiocyanate.
Thiocyanate shares its negative charge approximately equally betweensulfur andnitrogen. As a consequence, thiocyanate can act as anucleophile at either sulfur or nitrogen—it is anambidentate ligand. [SCN]− can also bridge two (M−SCN−M) or even three metals (>SCN− or −SCN<). Experimental evidence leads to the general conclusion thatclass A metals (hard acids) tend to formN-bonded thiocyanate complexes, whereasclass B metals (soft acids) tend to formS-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.[17] It [SCN] is considered as a weak ligand. ([NCS] is a strong ligand)[18]
The blood-red colored (up) complex [Fe(NCS)(H2O)5]2+ (left), indicates the presence of Fe3+ in solution.
If [SCN]− is added to a solution withiron(III) ions, a blood-red solution forms mainly due to the formation of[Fe(NCS)(H2O)5]2+, i.e. pentaaqua(thiocyanato-N)iron(III). Lesser amounts of other hydrated compounds also form: e.g. Fe(SCN)3 and [Fe(SCN)4]−.[19]
Similarly, Co2+ gives a blue complex with thiocyanate.[20] Both the iron and cobalt complexes can be extracted into organic solvents like diethyl ether or amyl alcohol. This allows the determination of these ions even in strongly coloured solutions. The determination of Co(II) in the presence of Fe(III) is possible by adding KF to the solution, which forms uncoloured, very stable complexes with Fe(III), which no longer react with SCN−.[21]
Phospholipids or some detergents aid the transfer of thiocyanatoiron into chlorinated solvents like chloroform and can be determined in this fashion.[22]
^Wilson, I. R.; Harris, G. M. (January 1, 1961). "The oxidation of thiocyanate ion by hydrogen peroxide II: The acid-catalyzed reaction".Journal of the American Chemical Society.83 (2):286–289.doi:10.1021/ja01463a007.
^Katayama, Yoko; Hashimoto, Kanako; Nakayama, Hiroshi; Mino, Hiroyuki; Nojiri, Masaki; Ono, Taka-aki; Nyunoya, Hiroshi; Yohda, Masafumi; Takio, Koji; Odaka, Masafumi (2006). "Thiocyanate Hydrolase is a Cobalt-Containing Metalloenzyme with a Cysteine-Sulfinic Acid Ligand".Journal of the American Chemical Society.128 (3):728–729.doi:10.1021/ja057010q.PMID16417356.
^"Hypothyroidism".umm.edu. University of Maryland Medical Center. Retrieved3 December 2014.
^Warren F. Gorman; Emanuel Messinger; And Morris Herman (1949). "Toxicity of Thiocyanates Used in Treatment of Hypertension".Ann Intern Med.30 (5):1054–1059.doi:10.7326/0003-4819-30-5-1054.PMID18126744.
^Palenik, Gus J.; Clark, George Raymond (1970). "Crystal and molecular structure of isothiocyanatothiocyanato-(1-diphenylphosphino-3-dimethylaminopropane)palladium(II)".Inorganic Chemistry.9 (12):2754–2760.doi:10.1021/ic50094a028.ISSN0020-1669.
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