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°.

CSD CIF YUNGEW.png|Crystal structure of [Ni^II(NCS)₆]^4-, a homoleptic complex of six isothiocyanate ligands. Color code: blue = N, yellow = S.

PalenikPdPN SCN NCSic1970.svg|Structure of Pd(Me₂N(CH₂)₃PPh₂)(SCN)(NCS) illustrating linkage isomerism of the SCN⁻ ligand.{{cite journal|last1=Palenik|first1=Gus J.|last2=Clark|first2=George Raymond|title=Crystal and Molecular Structure of Isothiocyanatothiocyanato-(1-diphenylphosphino-3-dimethylaminopropane)palladium(II)|journal=Inorganic Chemistry|volume=9|issue=12|year=1970|pages=2754–2760|issn=0020-1669|doi=10.1021/ic50094a028}}

CSD CIF OGAKOX.png|Crystal structure of [Re^IV(NCS)₅(SCN)]^2-. Color code: blue = N, yellow = S.

(Ni2(SCN)8)4-.svg|Structure of the dinuclear complex [Ni^II₂(SCN)₈]^4- with a bridging SCN⁻ ligand.

Most homoleptic complexes of NCS⁻ feature isothiocyanate ligands (N-bonded). All first-row metals bind thiocyanate in this way.{{cite journal |doi=10.1021/ic401558f |title=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 |date=2013 |last1=Shurdha |first1=Endrit |last2=Moore |first2=Curtis E. |last3=Rheingold |first3=Arnold L. |last4=Lapidus |first4=Saul H. |last5=Stephens |first5=Peter W. |last6=Arif |first6=Atta M. |last7=Miller |first7=Joel S. |journal=Inorganic Chemistry |volume=52 |issue=18 |pages=10583–10594 |pmid=23981238 }} 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).{{cite journal |doi=10.1002/ejic.200400867 |title=The Hexakis(thiocyanato)ferrate(III) Ion: A Coordination Chemistry Classic Reveals an Interesting Geometry Pattern for the Thiocyanate Ligands |date=2005 |last1=Addison |first1=Anthony W. |last2=Butcher |first2=Raymond J. |last3=Homonnay |first3=Zoltán |last4=Pavlishchuk |first4=Vitaly V. |last5=Prushan |first5=Michael J. |last6=Thompson |first6=Laurence K. |journal=European Journal of Inorganic Chemistry |issue=12 |pages=2404–2408 }} 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-.

Few homoleptic complexes of NCS⁻ feature thiocyanate ligands (S-bonded). Octahedral complexes include [M(SCN)₆]^3- (M = Rh{{cite journal |doi=10.1002/zaac.19956210623 |title=Darstellung und Kristallstruktur von Tetraphenylphosphonium‐Hexathiocyanatorhodat(III), [P(C₆H₅)₄]₃[Rh(SCN)₆] |date=1995 |last1=Vogt |first1=J.‐U. |last2=Haeckel |first2=O. |last3=Preetz |first3=W. |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=621 |issue=6 |pages=1033–1036 }} and Ir{{cite journal |doi=10.1002/(SICI)1521-3749(199808)624:8<1319::AID-ZAAC1319>3.0.CO;2-Q |title=Kristallstruktur von (Me₄N)₃[Ir(SCN)₆], Schwingungsspektrum und Normalkoordinatenanalyse |date=1998 |last1=Rohde |first1=J.-U. |last2=Preetz |first2=W. |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=624 |issue=8 |pages=1319–1323 }}) and [Pt(SCN)₆]^2-. Square planar complexes include [M(SCN)₄]^z- (M = Pd(II), Pt(II),{{cite journal |doi=10.1002/(SICI)1521-3749(200004)626:4<905::AID-ZAAC905>3.3.CO;2-Q |title=Kristallstrukturen, Spektroskopische Charakterisierung und Normalkoordinatenanalyse von (n-Bu₄N)₂[M(ECN)₄] (M = Pd, Pt; E = S, Se) |date=2000 |last1=Rohde |first1=J.-U. |last2=Malottki |first2=B. von |last3=Preetz |first3=W. |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=626 |issue=4 |pages=905–910 }} 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.{{cite journal |doi=10.1002/1521-3749(200104)627:4<615::AID-ZAAC615>3.0.CO;2-4 |title=Kristallstrukturen, Schwingungsspektren und Normalkoordinatenanalyse von (n-Bu₄N)₂[Os(NCS)₆] und (n-Bu₄N)₃[Os(NCS)₆] |date=2001 |last1=Stähler |first1=O. |last2=Preetz |first2=W. |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=627 |issue=4 |pages=615–619 }}

{{image frame|content=|align=center|width=250|caption=Resonance structures of the thiocyanate ion}}

Thiocyanate shares its negative charge approximately equally between sulfur and nitrogen.{{cite journal |doi=10.1016/0010-8545(90)80019-P |title=Ambidentate Ligands, the Schizophrenics of Coordination Chemistry |date=1990 |last1=Burmeister |first1=J. |journal=Coordination Chemistry Reviews |volume=105 |pages=77–133 }} 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)]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⁺.{{cite journal |doi=

10.1016/0010-8545(94)80078-2|title=

The Linkage Isomerism of Thiocyanate Bonded to Cobalt(III)|date=

1994|last1=

Buckingham|first1=

D.A.|journal=

Coordination Chemistry Reviews|volume=

135-136|pages=

587–621}}

:{{chem2|[Co(NH3)5(H2O)](3+) + SCN- -> [Co(NH3)5(SCN)](2+) + H2O}}

:{{chem2|[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),{{cite journal |doi=10.1002/zaac.19966221123 |title=Darstellung und Kristallstruktur von (n-Bu₄N)₃[Ir(NCS)(SCN)₅] |date=1996 |last1=Semrau |first1=M. |last2=Preetz |first2=W. |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=622 |issue=11 |pages=1953–1956 }} and Re(IV).{{cite journal |doi=10.1016/j.ica.2008.01.017 |title=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)] |date=2008 |last1=González |first1=Ricardo |last2=Barboza |first2=Natalia |last3=Chiozzone |first3=Raúl |last4=Kremer |first4=Carlos |last5=Armentano |first5=Donatella |last6=De Munno |first6=Giovanni |last7=Faus |first7=Juan |journal=Inorganica Chimica Acta |volume=361 |issue=9–10 |pages=2715–2720 }}

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-.{{cite journal |doi=10.1139/v03-114 |title=Crystal Structures, Magnetic Properties, and Absorption Spectra of Nickel(II) Thiocyanato Complexes: A Comparison of Different Coordination Geometries |date=2003 |last1=Larue |first1=Bruno |last2=Tran |first2=Lan-Tâm |last3=Luneau |first3=Dominique |last4=Reber |first4=Christian |journal=Canadian Journal of Chemistry |volume=81 |issue=11 |pages=1168–1179 }}

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-.{{cite journal |doi=10.1002/zaac.19966220916 |title=Darstellung und Kristallstruktur von trans ‐(Ph₄As)₂[OsCl₂(NCS)₂(SCN)₂], Schwingungsspektren und Normalkoordinatenanalyse |date=1996 |last1=Semrau |first1=M. |last2=Preetz |first2=W. |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=622 |issue=9 |pages=1537–1541 }} Reinecke's salt, a precipitating agent, is a derivative of [Cr(NCS)₄(NH₃)₂]⁻.

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 identity of the chromophore remains unknown.{{cite book|title=The Iron(III) Thiocyanate Reaction: Research History and Role in Chemical Analysis|date=2019|last=de Berg|first=Kevin C.|publisher=Springer|isbn=978-3-030-27316-3}} 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.{{cite book|author=Berzelius J. J.| year=1826|title=Lehrbuch der Chemie|publisher=Arnoldischen Buchhandlung|location=Dresden}} 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:{{cite journal |doi=10.1021/ja057010q |title=Thiocyanate Hydrolase is a Cobalt-Containing Metalloenzyme with a Cysteine-Sulfinic Acid Ligand |date=2006 |last1=Katayama |first1=Yoko |last2=Hashimoto |first2=Kanako |last3=Nakayama |first3=Hiroshi |last4=Mino |first4=Hiroyuki |last5=Nojiri |first5=Masaki |last6=Ono |first6=Taka-aki |last7=Nyunoya |first7=Hiroshi |last8=Yohda |first8=Masafumi |last9=Takio |first9=Koji |last10=Odaka |first10=Masafumi |journal=Journal of the American Chemical Society |volume=128 |issue=3 |pages=728–729 |pmid=16417356 }}

:{{chem2|SCN- + H2O + H+ -> SCO + NH3}}

A copper-containing thiocyanate hydrolase catalyzes its conversion to cyanate:{{cite journal|doi=10.1073/pnas.1922133117 |title=Trinuclear Copper Biocatalytic Center Forms an Active Site of Thiocyanate Dehydrogenase |date=2020 |last1=Tikhonova |first1=Tamara V. |last2=Sorokin |first2=Dimitry Y. |last3=Hagen |first3=Wilfred R. |last4=Khrenova |first4=Maria G. |last5=Muyzer |first5=Gerard |last6=Rakitina |first6=Tatiana V. |last7=Shabalin |first7=Ivan G. |last8=Trofimov |first8=Anton A. |last9=Tsallagov |first9=Stanislav I. |last10=Popov |first10=Vladimir O. |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=10 |pages=5280–5290 |doi-access=free |pmid=32094184 |bibcode=2020PNAS..117.5280T |pmc=7071890 }}

:{{chem2|SCN- + H2O -> OCN- + H2S}}

In both cases, metal-SCN complexes are invoked as intermediates.

Almost all thiocyanate complexes are prepared from thiocyanate salts using ligand substitution reactions.{{cite book |doi=10.1002/9780470132333.ch61 |chapter=cis -Dichlorobis(ethylenediamine)-chromium(III) Chloride and Trans -Bis-(thiocyanato)Bis(ethylenediamine)Chromium(III) Thiocyanate |title=Inorganic Syntheses |date=1946 |last1=Rollinson |first1=Carl L. |last2=Bailar |first2=John C. |volume=2 |pages=200–202 |isbn=978-0-470-13161-9 }}{{cite book |doi=10.1002/9780470132388.ch56 |chapter=Inner Complexes of Cobalt(III) with Diethylenetriamine |title=Inorganic Syntheses |date=1963 |last1=Crayton |first1=Philip H. |volume=7 |pages=207–213 |isbn=978-0-470-13166-4 }} 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:{{cite journal |doi=10.1016/S0022-328X(00)80741-X |title=Activation of Thiocyanogen and Selenocyanogen by Low Oxidation State Transition Metal Complexes |date=1976 |last1=Faraone |first1=Felice |last2=Sergi |first2=Sergio |journal=Journal of Organometallic Chemistry |volume=112 |issue=2 |pages=201–207 }}

:{{chem2|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).{{OrgSynth |last=Dakin |first=H. D. |title=Reinecke Salt |volume=15 |page=74 |year=1935 |doi=10.15227/orgsyn.015.0074}}

• {{cite journal |doi=10.1016/0010-8545(94)01121-q |title=Bonding Properties of Thiocyanate Groups in Copper(II) and Copper(I) Complexes |date=1995 |last1=Kabešová |first1=M. |last2=Boča |first2=R. |last3=Melník |first3=M. |last4=Valigura |first4=D. |last5=Dunaj-Jurčo |first5=M. |journal=Coordination Chemistry Reviews |volume=140 |pages=115–135 }}
• {{cite journal |doi=10.1351/pac199769071489 |title=Critical Survey of Stability Constants of Complexes of Thiocyanate Ion (Technical Report) |date=1997 |last1=Bahta |first1=Abraha |last2=Parker |first2=G. A. |last3=Tuck |first3=D. G. |journal=Pure and Applied Chemistry |volume=69 |issue=7 |pages=1489–1548 }}

{{Coordination complexes}}

Category:Inorganic chemistry

Category:Transition metals

Category:Coordination chemistry