Transition metal isocyanide complexes

According to the Covalent bond classification method, isocyanides are classified as L ligands, i.e., charge-neutral Lewis bases. With respect to HSAB theory, it is classified as soft.

Compared to CO, most isocyanides are superior Lewis bases and weaker pi-acceptors. Trifluoromethylisocyanide is the exception, its coordination properties are very similarly to those of CO. Isocyanide complexes often mirror the stoichiometry and structures of metal carbonyls. Like CO, isocyanides engage in pi-backbonding. The M-C-N angle provides some measure of the degree of backbonding. In electron-rich complexes, this angle is usually deviates from 180°. Unlike CO, cationic and dicationic complexes are common. RNC ligands are typically terminal, but bridging RNC ligands are common. Bridging isocyanides are always bent. General trends can be appreciated by inspection of the homoleptic complexes of the first row transition metals.

Because the CNC linkage is linear, the cone angle of these ligands is small, so it is easy to prepare polyisocyanide complexes. Many complexes of isocyanides show high coordination numbers, e.g. the eight-coordinate cation {{chem2|[Nb(CNBu\st)6I2](+)}}.{{cite journal |doi=10.1021/om960510v|title=Niobium-Centered C−C Coupling of Isonitriles|year=1996|last1=Collazo|first1=César|last2=Rodewald|first2=Dieter|last3=Schmidt|first3=Hauke|last4=Rehder|first4=Dieter|journal=Organometallics|volume=15|issue=22|pages=4884–4887}} Very bulky isocyanide ligands are also known, e.g. C₆H₃-2,6-Ar₂-NC (Ar =aryl).{{cite journal |doi=10.1126/science.aaw6102|title=Terminal coordination of diatomic boron monofluoride to iron|year=2019|last1=Drance|first1=Myles J.|last2=Sears|first2=Jeffrey D.|last3=Mrse|first3=Anthony M.|last4=Moore|first4=Curtis E.|last5=Rheingold|first5=Arnold L.|last6=Neidig|first6=Michael L.|last7=Figueroa|first7=Joshua S.|s2cid=78094683|journal=Science|volume=363|issue=6432|pages=1203–1205|pmid=30872521|bibcode=2019Sci...363.1203D|doi-access=free}}

File:Structure of Os3(CO)9((CNCH2)3CMe, VIGYUF.png

Di- and triisocyanide ligands are well developed, e.g., (CH₂)_n(NC)₂. Usually steric factors force these ligands to bind to two separate metals, i.e., they are binucleating ligands.{{cite journal |doi=10.1016/S0010-8545(00)00415-X|title=Chemistry, Properties and Applications of the Assembling 1,8-Diisocyano-p-menthane, 2,5-Dimethyldiisocyanohexane and 1,3-Diisocyanopropane Ligands and Their Coordination Polynuclear Complexes|year=2001|last1=Harvey|first1=P.|journal=Coordination Chemistry Reviews|volume=219-221|pages=17–52}} Chelating diisocyanide ligands require elaborate backbones.{{cite journal |doi=10.1021/ic00168a048|title=Synthesis and characterization of homoleptic complexes of the chelating bidentate isocyano ligand tert-BuDiNC|year=1983|last1=Plummer|first1=Daniel T.|last2=Angelici|first2=Robert J.|author2-link=Robert Angelici|journal=Inorganic Chemistry|volume=22|issue=26|pages=4063–4070}}

File:Fe(t-BuNC)5 (PTBICF10).png

Because of their low steric profile and high basicity, isocyanide ligands often install easily, e.g. by treating metal halides with the isocyanide. Many metal cyanides can be N-alkylated to give isocyanide complexes.{{cite journal |doi=10.1021/cr00019a016|title=Emergence of a CNH and cyano complex based organometallic chemistry|year=1993|last1=Fehlhammer|first1=Wolf P.|last2=Fritz|first2=Marcus.|journal=Chemical Reviews|volume=93|issue=3|pages=1243–1280}}

File:Chugaev's Carbene.svg, was not recognized as such until decades after its preparation.{{cite journal | vauthors = Hahn FE, Jahnke MC | title = Heterocyclic Carbenes: Synthesis and Coordination Chemistry | journal = Angewandte Chemie International Edition | volume = 47 | issue = 17 | pages = 3122–72 | date = 2008 | pmid = 18398856 | doi = 10.1002/anie.200703883 }}]]

Typically, isocyanides are spectator ligands, but their reduced and oxidized complexes can prove reactive by virtue of the unsaturated nature of the ligand

Cationic isocyanide complexes are susceptible to nucleophilic attack at carbon. In this way, the first metal carbene complexes where prepared.

Because isocyanides are more basic donors ligands than CO, their complexes are susceptible to oxidation and protonation. Thus, {{chem2|Fe(tBuNC)5}} is easily protonated, whereas its counterpart {{chem2|Fe(CO)5}} is not:{{cite journal |doi=10.1039/DT9800001789|title=Notes. Protonation of Pentakis(t-butyl Isocyanide)iron|year=1980|last1=Bassett|first1=Jean-Maria|last2=Farrugia|first2=Louis J.|last3=Stone|first3=F. Gordon A.|journal=Journal of the Chemical Society, Dalton Transactions|issue=9|page=1789}}

:Fe(CNR)₅ + H⁺ → [HFeL₅]⁺

:Fe(CO)₅ + H⁺ → no reaction

Some electron-rich isocyanide complexes protonate at N to give aminocarbyne complexes:{{cite journal |doi=10.1016/S0022-328X(00)00641-0|title=Coordination Chemistry of CNH₂, The Simplest Aminocarbyne|year= 2001|last1=Pombeiro|first1=Armando J.L|last2=Fátima|first2=M.|last3=Guedes Da Silva|first3=C.|journal=Journal of Organometallic Chemistry|volume=617-618|pages=65–69}}

:L_nM-CNR + H⁺ → [L_nM≡CN(H)R]⁺

Isocyanides sometimes insert into metal-alkyl bonds to form iminoacyls.{{cite journal| vauthors =Vicente J, Abad JA, Fortsch W, Lopez-Saez MJ |title=Reactivity of ortho-Palladated Phenol Derivatives with Unsaturated Molecules|journal=Organometallics|year= 2004|volume=23|pages=4414–4429|doi=10.1021/om0496131}}

Because isocyanides are both acceptors and donors, they exhibit more reversible redox than metal carbonyls. This aspect is illustrated by the isolation of the homoleptic vanadium hexaisocyanide complex in three oxidation states, i.e., [V(CNC₆H₃-2,6-Me₂)₆]ⁿ for n = -1, 0, +1.{{cite journal |doi=10.1021/ja000212w|title=First Paramagnetic Zerovalent Transition Metal Isocyanides. Syntheses, Structural Characterizations, and Magnetic Properties of Novel Low-Valent Isocyanide Complexes of Vanadium1|year=2000|last1=Barybin|first1=Mikhail V.|last2=Young|first2=Victor G.|last3=Ellis|first3=John E.|journal=Journal of the American Chemical Society|volume=122|issue=19|pages=4678–4691}}

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The ν_C≡N band in isocyanides is intense in the range of 2165–2110 cm⁻¹.{{cite journal | last = Stephany | first = R. W. |author2=de Bie, M. J. A. |author3=Drenth, W. | title = A ¹³C-NMR and IR study of isocyanides and some of their complexes | journal = Organic Magnetic Resonance | year = 1974 | volume = 6 | issue = 1 | pages = 45–47 | doi = 10.1002/mrc.1270060112}} The value of ν_C≡N is diagnostic of the electronic character of the complex. In complexes where RNC is primarily a sigma donor ligand, ν_C≡N shifts to higher energies vs free isocyanide. Thus, for {{chem2|[Co(CN\st\sBu)5]+}}, ν_C≡N = 2152, 2120 cm^−l. In contrast, for the electron-rich species Fe₂(CNEt)₉, ν_C≡N = 2060, 1920, 1701, 1652 cm^−l.{{cite journal |doi=10.1039/C39780001000|title=Formation of Nona(ethyl isocyanide)di-iron from Penta(ethyl isocyanide)iron and Reaction of Penta(t-butyl isocyanide)iron with Diphenylacetylene; X-ray Crystal Structures of Nona(ethyl Isocyanide)di-iron and Tris(t-butyl isocyanide)]1,4-bis-(t-butylimino)-2,3-diphenylbuta-1,3-diene]iron|year=1978|last1=Bassett|first1=Jean-Marie|last2=Green|first2=Michael|last3=Howard|first3=Judith A. K.|last4=Stone|first4=F. Gordon A.|journal=Journal of the Chemical Society, Chemical Communications|issue=22|page=1000}}

• Cyanometalate - coordination compounds containing cyanide ligands (coordinating via C)
• Transition metal nitrile complexes - coordination compounds containing nitrile ligands, which are isomers of isonitriles

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{{Coordination complexes}}

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Category:Coordination complexes

Category:Isocyanides