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

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°.

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

Homoleptic complexes

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
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

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

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

Thiocyanate complexes are not widely used. Copper(I) thiocyanate is a reagent for the Sandmeyer reaction.

Further reading
  • 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
  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. 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. 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. 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.
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