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Transition metal complexes of thiocyanate

Transition metal complexes of thiocyanate describes coordination complexes containing one or more thiocyanate (SCN-) ligands. The topic also includes transition metal complexes of isothiocyanate. These complexes have few applications but played significant role in the development of coordination chemistry.

Structure and bonding

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Hard metal cations, as classified by HSAB theory, tend to form N-bonded complexes (isothiocyanates), whereas class B or soft metal cations tend to form S-bonded thiocyanate complexes. For the isothiocyanates, the M-N-C angle is usually close to 180°. For the thiocyanates, the M-S-C angle is usually close to 100°.

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    Crystal structure of [NiII(NCS)6]4-, a homoleptic complex of six isothiocyanate ligands. Color code: blue = N, yellow = S.
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    Structure of Pd(Me2N(CH2)3PPh2)(SCN)(NCS) illustrating linkage isomerism of the SCN- ligand..[1]
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    Crystal structure of [ReIV(NCS)5(SCN)]2-.[2] Color code: blue = N, yellow = S.
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    Structure of the dinuclear complex [NiII2(SCN)8]4- with a bridging SCN- ligand.

Homoleptic complexes

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Most homoleptic complexes of NCS- feature isothiocyanate ligands (N-bonded). All first-row metals bind thiocyanate in this way.[3] Octahedral complexes [M(NCS)6]z- include M = Ti(III), Cr(III), Mn(II), Fe(III), Ni(II), Mo(III), Tc(IV), and Ru(III). Four-coordinated tetrakis(isothiocyanate) complexes would be tetrahedral since isothiocyanate is a weak-field ligand. Two examples are the deep blue [Co(NCS)4]2- and the green [Ni(NCS)4]2-.[4]

Few homoleptic complexes of NCS- feature thiocyanate ligands (S-bonded). Octahedral complexes include [M(SCN)6]3- (M = Rh[5] and Ir[6]) and [Pt(SCN)6]2-. Square planar complexes include [M(SCN)4]z- (M = Pd(II), Pt(II),[7] and Au(III)). Colorless [Hg(SCN)4]2- is tetrahedral.

Some octahedral isothiocyanate complexes undergo redox reactions reversibly. Orange [Os(NCS)6]3- can be oxidized to violet [Os(NCS)6]2-. The Os-N distances in both derivatives are almost identical at 200 picometers.[8]

Linkage isomerism

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Resonance structures of the thiocyanate ion

Thiocyanate shares its negative charge approximately equally between sulfur and nitrogen.[9] Thiocyanate can bind metals at either sulfur or nitrogen — it is an ambidentate ligand. Other factors, e.g. kinetics and solubility, sometimes influence the observed isomer. For example, [Co(NH3)5(NCS)]+ is the thermodynamic isomer, but [Co(NH3)5(SCN)]2+ forms as the kinetic product of the reaction of thiocyanate salts with [Co(NH3)5(H2O)]3+.[10]

[Co(NH3)5(H2O)]3+ + SCN− → [Co(NH3)5(SCN)]2+ + H2O
[Co(NH3)5(SCN)]2+ → [Co(NH3)5(NCS)]2+

Some complexes of SCN- feature both but only thiocyanate and isothiocyanate ligands. Examples are found for heavy metals in the middle of the d-period: Ir(III),[11] and Re(IV).[2]

SCN-bridged complexes

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As a ligand, [SCN]− can also bridge two (M−SCN−M) or even three metals (>SCN− or −SCN<). One example of an SCN-bridged complex is [Ni2(SCN)8]4-.[4]

Mixed ligand complexes

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This article focuses on homoleptic complexes, which are simpler to describe and analyze. Most complexes of SCN-, however are mixed ligand species. Mentioned above is one example, [Co(NH3)5(NCS)]2+. Another example is [OsCl2(SCN)2(NCS)2]2-.[12] Reinecke's salt, a precipitating agent, is a derivative of [Cr(NCS)4(NH3)2]-.

Applications and occurrence

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Thiocyanate complexes are not widely used commercially. Possibly the oldest application of thiocyanate complexes was the use of thiocyanate as a test for ferric ions in aqueous solution.[13] The reverse was also used: testing for the presence of thiocyanate by the addition of ferric salts. The 1:1 complex of thiocyanate and iron is deeply red. The effect was first reported in 1826.[14] The structure of this species has never been confirmed by X-ray crystallography. The test is largely archaic.

Copper(I) thiocyanate is a reagent for the conversion of aryl diazonium salts to arylthiocyanates, a version of the Sandmeyer reaction.

Since thiocyanate occurs naturally, it is to be expected that it serves as a substrate for enzymes. Two metalloenzymes, thiocyanate hydrolases, catalyze the hydrolysis of thiocyanate. A cobalt-containing hydrolase catalyzes its conversion to carbonyl sulfide:[15]

SCN− + H2O + H+ → SCO + NH3

A copper-containing thiocyanate hydrolase catalyzes its conversion to cyanate:[16]

SCN− + H2O → OCN− + H2S

In both cases, metal-SCN complexes are invoked as intermediates.

Synthesis

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Almost all thiocyanate complexes are prepared from thiocyanate salts using ligand substitution reactions.[10][17][18] Typical thiocyanate sources include ammonium thiocyanate and potassium thiocyanate.

An unusual route to thiocyanate complexes involves oxidative addition of thiocyanogen to low valent metal complexes:[19]

Ru(PPh3)2(CO)3 + (SCN)2 → Ru(NCS)2(PPh3)2(CO)2 + CO, where Ph = C6H5

Even though the reaction involves cleavage of the S-S bond in thiocyanogen, the product is the Ru-NCS linkage isomer.

In another unusual method, thiocyanate functions as both a ligand and as a reductant in its reaction with dichromate to give [Cr(NCS)4(NH3)2]-. In this conversion, Cr(VI) converts to Cr(III).[20]

Further reading

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  • Kabešová, M.; Boča, R.; Melník, M.; Valigura, D.; Dunaj-Jurčo, M. (1995). "Bonding Properties of Thiocyanate Groups in Copper(II) and Copper(I) Complexes". Coordination Chemistry Reviews. 140: 115–135. doi:10.1016/0010-8545(94)01121-q.
  • Bahta, Abraha; Parker, G. A.; Tuck, D. G. (1997). "Critical Survey of Stability Constants of Complexes of Thiocyanate Ion (Technical Report)". Pure and Applied Chemistry. 69 (7): 1489–1548. doi:10.1351/pac199769071489.

References

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  1. ^ 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. ISSN 0020-1669.
  2. ^ a b González, Ricardo; Barboza, Natalia; Chiozzone, Raúl; Kremer, Carlos; Armentano, Donatella; De Munno, Giovanni; Faus, Juan (2008). "Linkage Isomerism in the Metal Complex Hexa(thiocyanato)rhenate(IV): Synthesis and Crystal Structure of (NBu4)2[Re(NCS)6] and [Zn(NO3)(Me2phen)2]2[Re(NCS)5(SCN)]". Inorganica Chimica Acta. 361 (9–10): 2715–2720. doi:10.1016/j.ica.2008.01.017.
  3. ^ Shurdha, Endrit; Moore, Curtis E.; Rheingold, Arnold L.; Lapidus, Saul H.; Stephens, Peter W.; Arif, Atta M.; Miller, Joel S. (2013). "First Row Transition Metal(II) Thiocyanate Complexes, and Formation of 1-, 2-, and 3-Dimensional Extended Network Structures of M(NCS)2(Solvent)2 (M = Cr, Mn, Co) Composition". Inorganic Chemistry. 52 (18): 10583–10594. doi:10.1021/ic401558f. PMID 23981238.
  4. ^ a b Larue, Bruno; Tran, Lan-Tâm; Luneau, Dominique; Reber, Christian (2003). "Crystal Structures, Magnetic Properties, and Absorption Spectra of Nickel(II) Thiocyanato Complexes: A Comparison of Different Coordination Geometries". Canadian Journal of Chemistry. 81 (11): 1168–1179. doi:10.1139/v03-114.
  5. ^ Vogt, J.‐U.; Haeckel, O.; Preetz, W. (1995). "Darstellung und Kristallstruktur von Tetraphenylphosphonium‐Hexathiocyanatorhodat(III), [P(C6H5)4]3[Rh(SCN)6]". Zeitschrift für Anorganische und Allgemeine Chemie. 621 (6): 1033–1036. doi:10.1002/zaac.19956210623.
  6. ^ Rohde, J.-U.; Preetz, W. (1998). "Kristallstruktur von (Me4N)3[Ir(SCN)6], Schwingungsspektrum und Normalkoordinatenanalyse". Zeitschrift für Anorganische und Allgemeine Chemie. 624 (8): 1319–1323. doi:10.1002/(SICI)1521-3749(199808)624:8<1319::AID-ZAAC1319>3.0.CO;2-Q.
  7. ^ Rohde, J.-U.; Malottki, B. von; Preetz, W. (2000). "Kristallstrukturen, Spektroskopische Charakterisierung und Normalkoordinatenanalyse von (n-Bu4N)2[M(ECN)4] (M = Pd, Pt; E = S, Se)". Zeitschrift für Anorganische und Allgemeine Chemie. 626 (4): 905–910. doi:10.1002/(SICI)1521-3749(200004)626:4<905::AID-ZAAC905>3.3.CO;2-Q.
  8. ^ Stähler, O.; Preetz, W. (2001). "Kristallstrukturen, Schwingungsspektren und Normalkoordinatenanalyse von (n-Bu4N)2[Os(NCS)6] und (n-Bu4N)3[Os(NCS)6]". Zeitschrift für Anorganische und Allgemeine Chemie. 627 (4): 615–619. doi:10.1002/1521-3749(200104)627:4<615::AID-ZAAC615>3.0.CO;2-4.
  9. ^ Burmeister, J. (1990). "Ambidentate Ligands, the Schizophrenics of Coordination Chemistry". Coordination Chemistry Reviews. 105: 77–133. doi:10.1016/0010-8545(90)80019-P.
  10. ^ a b Buckingham, D.A. (1994). "The Linkage Isomerism of Thiocyanate Bonded to Cobalt(III)". Coordination Chemistry Reviews. 135–136: 587–621. doi:10.1016/0010-8545(94)80078-2.
  11. ^ Semrau, M.; Preetz, W. (1996). "Darstellung und Kristallstruktur von (n-Bu4N)3[Ir(NCS)(SCN)5]". Zeitschrift für Anorganische und Allgemeine Chemie. 622 (11): 1953–1956. doi:10.1002/zaac.19966221123.
  12. ^ Semrau, M.; Preetz, W. (1996). "Darstellung und Kristallstruktur von trans ‐(Ph4As)2[OsCl2(NCS)2(SCN)2], Schwingungsspektren und Normalkoordinatenanalyse". Zeitschrift für Anorganische und Allgemeine Chemie. 622 (9): 1537–1541. doi:10.1002/zaac.19966220916.
  13. ^ de Berg, Kevin C. (2019). The Iron(III) Thiocyanate Reaction: Research History and Role in Chemical Analysis. Springer. ISBN 978-3-030-27316-3.
  14. ^ Berzelius J. J. (1826). Lehrbuch der Chemie. Dresden: Arnoldischen Buchhandlung.
  15. ^ 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. PMID 16417356.
  16. ^ Tikhonova, Tamara V.; Sorokin, Dimitry Y.; Hagen, Wilfred R.; Khrenova, Maria G.; Muyzer, Gerard; Rakitina, Tatiana V.; Shabalin, Ivan G.; Trofimov, Anton A.; Tsallagov, Stanislav I.; Popov, Vladimir O. (2020). "Trinuclear Copper Biocatalytic Center Forms an Active Site of Thiocyanate Dehydrogenase". Proceedings of the National Academy of Sciences. 117 (10): 5280–5290. Bibcode:2020PNAS..117.5280T. doi:10.1073/pnas.1922133117. PMID 32094184.
  17. ^ Rollinson, Carl L.; Bailar, John C. (1946). "cis -Dichlorobis(ethylenediamine)-chromium(III) Chloride and Trans -Bis-(thiocyanato)Bis(ethylenediamine)Chromium(III) Thiocyanate". Inorganic Syntheses. Vol. 2. pp. 200–202. doi:10.1002/9780470132333.ch61. ISBN 978-0-470-13161-9.
  18. ^ Crayton, Philip H. (1963). "Inner Complexes of Cobalt(III) with Diethylenetriamine". Inorganic Syntheses. Vol. 7. pp. 207–213. doi:10.1002/9780470132388.ch56. ISBN 978-0-470-13166-4.
  19. ^ Faraone, Felice; Sergi, Sergio (1976). "Activation of Thiocyanogen and Selenocyanogen by Low Oxidation State Transition Metal Complexes". Journal of Organometallic Chemistry. 112 (2): 201–207. doi:10.1016/S0022-328X(00)80741-X.
  20. ^ Dakin, H. D. (1935). "Reinecke Salt". Organic Syntheses. 15: 74. doi:10.15227/orgsyn.015.0074.
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