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.

Contents

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°.

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

Related Research Articles

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<span class="mw-page-title-main">Cyanate</span> Anion with formula OCN and charge –1

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<span class="mw-page-title-main">Reinecke's salt</span> Chemical compound

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<span class="mw-page-title-main">Metal amides</span>

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References

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