Transition metal complexes of thiocyanate

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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[edit]

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.
    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]
    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.
    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.
    Structure of the dinuclear complex [NiII2(SCN)8]4- with a bridging SCN- ligand.

Homoleptic complexes[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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

Further reading[edit]

  • 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[edit]

  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. ^ 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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