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Transition metal complexes of thiocyanate

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 [Ni^II(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 [Re^IV(NCS)₅(SCN)]^2-.^[2] Color code: blue = N, yellow = S.
• 
  Structure of the dinuclear complex [Ni^II₂(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]

${\displaystyle {\ce {S=C=N^{\ominus }<->{^{\ominus }S}-C}}{\ce {#N}}}$

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 and occurrence[edit]

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[edit]

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[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]