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 a significant role in the development of coordination chemistry.[1]
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.](//upload.wikimedia.org/wikipedia/commons/thumb/e/e4/CSD_CIF_YUNGEW.png/120px-CSD_CIF_YUNGEW.png?utm_source=en.wikipedia.org&utm_campaign=parser&utm_content=thumbnail)
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.[2]![Crystal structure of [ReIV(NCS)5(SCN)]2−. Color code: blue = N, yellow = S.](//upload.wikimedia.org/wikipedia/commons/thumb/b/b4/CSD_CIF_OGAKOX.png/250px-CSD_CIF_OGAKOX.png?utm_source=en.wikipedia.org&utm_campaign=parser&utm_content=thumbnail)
Crystal structure of [ReIV(NCS)5(SCN)]2−.[3] Color code: blue = N, yellow = S.![Structure of the dinuclear complex [NiII2(SCN)8]4− with a bridging SCN− ligand.](//upload.wikimedia.org/wikipedia/commons/thumb/8/8c/%28Ni2%28SCN%298%294-.svg/250px-%28Ni2%28SCN%298%294-.svg.png?utm_source=en.wikipedia.org&utm_campaign=parser&utm_content=thumbnail)
Structure of the dinuclear complex [NiII2(SCN)8]4− with a bridging SCN− ligand.
![Crystal structure of [NiII(NCS)6]4−, a homoleptic complex of six isothiocyanate ligands. Color code: blue = N, yellow = S.](#/media/File:CSD_CIF_YUNGEW.png)
![Crystal structure of [ReIV(NCS)5(SCN)]2−. Color code: blue = N, yellow = S.](#/media/File:CSD_CIF_OGAKOX.png)
![Structure of the dinuclear complex [NiII2(SCN)8]4− with a bridging SCN− ligand.](#/media/File:(Ni2(SCN)8)4-.svg)
Homoleptic complexes
Most homoleptic complexes of NCS− feature isothiocyanate ligands (N-bonded). All first-row metals bind thiocyanate in this way.[4] 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).[5] 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−.[6]
Few homoleptic complexes of NCS− feature thiocyanate ligands (S-bonded). Octahedral complexes include [M(SCN)6]3− (M = Rh[7] and Ir[8]) and [Pt(SCN)6]2-. Square planar complexes include [M(SCN)4]z− (M = Pd(II), Pt(II),[9] 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.[10]
Linkage isomerism
Thiocyanate shares its negative charge approximately equally between sulfur and nitrogen.[11] Thiocyanate can bind metals at either sulfur or nitrogen — it is an ambidentate ligand. Other factors, such as kinetics and solubility, sometimes influence the observed isomer. For example, [Co(NH3)5(NCS)]2+ 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+.[12]
- [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 thiocyanate and isothiocyanate ligands. Examples are found for heavy metals in the middle of the d-period: Ir(III),[13] and Re(IV).[3]
Finally, the linkage isomers [Rh(SCN)6]3− and [Rh(SCN)5(NCS)]3− have been separated and individually characterized.[14]
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-.[6]
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-.[15] Reinecke's salt, a precipitating agent, is a derivative of [Cr(NCS)4(NH3)2]−.
Applications and occurrence
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. Addition of a thiocyanate salt to a solution containing ferric ions gives a deep red color. The reverse was also used: testing for the presence of thiocyanate by the addition of ferric salts. The identification of the chromophore in these tests remains complicated.[16] The 1:1 complex of thiocyanate and iron is deeply red. The effect was first reported in 1826.[17] 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.
Enzymatic substrate
Since it occurs naturally, thiocyanate unsurprisingly serves as a substrate for some enzymes. Two metalloenzymes, thiocyanate hydrolases, catalyze the hydrolysis of thiocyanate. A cobalt-containing hydrolase catalyzes its conversion to carbonyl sulfide:[18]
- SCN− + H2O + H+ → COS + NH3
A copper-containing thiocyanate hydrolase catalyzes its conversion to cyanate:[19]
- SCN− + H2O → OCN− + H2S
In both cases, metal–SCN complexes are invoked as intermediates. Although unlikely as a natural substrate, thiocyanate is a substrate for nitrogenase, being reduced to hydrogen sulfide, methane, and other products.[20]
Synthesis
Almost all thiocyanate complexes are prepared from thiocyanate salts using ligand substitution reactions.[12][21][22] 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:[23]
- 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).[24]
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.