Thiocyanate (also known as rhodanide) is the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

. It is the

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

of thiocyanic acid. Common derivatives include the colourless salts

potassium thiocyanate Potassium thiocyanate is the chemical compound with the molecular formula KSCN. It is an important salt of the thiocyanate anion, one of the pseudohalides. The compound has a low melting point relative to most other inorganic salts. Use in che ...

and

sodium thiocyanate Sodium thiocyanate (sometimes called sodium sulphocyanide) is the chemical compound with the formula NaSCN. This colorless deliquescent salt is one of the main sources of the thiocyanate anion. As such, it is used as a precursor for the synthe ...

. Mercury(II) thiocyanate was formerly used in pyrotechnics. Thiocyanate is analogous to the

cyanate Cyanate is an anion with the structural formula , usually written . It also refers to any salt containing it, such as ammonium cyanate. It is an isomer of the much less stable fulminate anion .William R. Martin and David W. Ball (2019): "Sma ...

ion, , wherein

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

is replaced by

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

. is one of the pseudohalides, due to the similarity of its reactions to that of

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a flu ...

ions. Thiocyanate used to be known as rhodanide (from a Greek word for

rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...

) because of the red colour of its complexes with

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

. Thiocyanate is produced by the reaction of elemental sulfur or

thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, e ...

with

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...

: : 8 CN- + S8 -> 8 SCN- : CN- + S2O3^2- -> SCN- + SO3^2- The second reaction is catalyzed by

thiosulfate sulfurtransferase Rhodanese, also known as rhodanase, thiosulfate sulfurtransferase, thiosulfate cyanide transsulfurase, and thiosulfate thiotransferase, ...

, a hepatic mitochondrial enzyme, and by other sulfur transferases, which together are responsible for around 80% of cyanide metabolism in the body.

Biological chemistry of thiocyanate in medicine

Thiocyanate is known to be an important part in the biosynthesis of hypothiocyanite by a

lactoperoxidase Lactoperoxidase is a peroxidase enzyme secreted from mammary, salivary and other mucosal glands including the lungs, bronchii and nose that functions as a natural and the first line of defense against bacteria and viruses. Lactoperoxidase is a ...

. Thus the complete absence of thiocyanate or reduced thiocyanate in the human body, (e.g.,

cystic fibrosis Cystic fibrosis (CF) is a rare genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. Ot ...

) is damaging to the human host defense system. Thiocyanate is a potent competitive inhibitor of the thyroid

sodium-iodide symporter The sodium/iodide cotransporter, also known as the sodium/iodide symporter (NIS), is a protein that in humans is encoded by the ''SLC5A5'' gene. It is a transmembrane glycoprotein with a molecular weight of 87 k Da and 13 transmembrane domains ...

. Iodine is an essential component of thyroxine. 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. In the early 20th century, thiocyanate was used in the treatment of

hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high b ...

, but it is no longer used because of associated toxicity. Sodium nitroprusside, a metabolite of which is thiocyanate, is however still used for the treatment of a hypertensive emergency. Rhodanese catalyzes the reaction of sodium nitroprusside with

to form the metabolite thiocyanate. Thiocyanate is also a metabolite of the detoxification of

s by Rhodanese.

Coordination chemistry

Thiocyanate shares its negative charge approximately equally between

and

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

. As a consequence, thiocyanate can act as a

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they ar ...

at either sulfur or nitrogen — it is an ambidentate ligand. CNsup>− 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 o(NH₃)₅(NCS)l₂ and o(NH₃)₅(SCN)l₂.It CNis considered as a weak ligand. ( NCS.html"_;"title="Isothiocyanate.html"_;"title="nowiki/>Isothiocyanate">NCS">Isothiocyanate.html"_;"title="nowiki/>Isothiocyanate">NCSis_a_strong_ligand)

_Test_for_iron(III)_and_cobalt(II)

If__CNsup>−_is_added_to_a_solution_with_ NCS.html"_;"title="Isothiocyanate.html"_;"title="nowiki/>Isothiocyanate">NCS">Isothiocyanate.html"_;"title="nowiki/>Isothiocyanate">NCSis_a_strong_ligand)

_Test_for_iron(III)_and_cobalt(II)

If__CNsup>−_is_added_to_a_solution_with_ferric">iron(III)_ions,_a_blood-red_solution_forms_mainly_due_to_the_formation_of_Thiocyanatoiron.html" ;"title="ferric.html" ;"title="Isothiocyanate">NCS.html" ;"title="Isothiocyanate.html" ;"title="nowiki/>Isothiocyanate">NCS">Isothiocyanate.html" ;"title="nowiki/>Isothiocyanate">NCSis a strong ligand)

Test for iron(III) and cobalt(II)

If CNsup>− is added to a solution with ferric">iron(III) ions, a blood-red solution forms mainly due to the formation of Thiocyanatoiron">[Fe(SCN)(H₂O)₅sup>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]⁻. Similarly, Co²⁺ gives a blue complex with thiocyanate. 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⁻. Phospholipids or some detergents aid the transfer of thiocyanatoiron into chlorinated solvents like chloroform and can be determined in this fashion.

References

Citations

{{Thyroid hormone receptor modulators Anions Sulfur ions Concrete admixtures

Biological chemistry of thiocyanate in medicine

Coordination chemistry

_Test_for_iron(III)_and_cobalt(II)

_Test_for_iron(III)_and_cobalt(II)

Test for iron(III) and cobalt(II)

See also

References

Citations