Transition metal isocyanide complexes arecoordination compounds containingisocyanide ligands. Several thousand isocyanides are known, but the coordination chemistry is dominated by a few ligands.[2] Common isonitrile ligands aremethyl isocyanide,tert-butyl isocyanide, phenyl isocyanide, and cyclohexylisocyanide.Some isocyanide complexes are used inmedical imaging.
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 inpi-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[Nb(CNBu−t)6I2]+.[3] Very bulky isocyanide ligands are also known, e.g. C6H3-2,6-Ar2-NC (Ar =aryl).[4]
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.[6] Chelating diisocyanide ligands require elaborate backbones.[7]
Structure of Fe(tert-BuNC)5. Notice that some C-N-C angles strongly deviate from 180°, a characteristic of low-valent isocyanide complexes.[8]
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.[9]
The first metal carbene complex,Chugaev's red salt, was not recognized as such until decades after its preparation.[10]
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,Fe(tBuNC)5 is easily protonated, whereas its counterpartFe(CO)5 is not:[8]
Fe(CNR)5 + H+ → [HFeL5]+
Fe(CO)5 + H+ → no reaction
Some electron-rich isocyanide complexes protonate at N to giveaminocarbyne complexes:[11]
LnM-CNR + H+ → [LnM≡CN(H)R]+
Isocyanides sometimes insert into metal-alkyl bonds to form iminoacyls.[12]
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.[13]
The νC≡N band in isocyanides is intense in the range of 2165–2110 cm−1.[27] The value of νC≡N is diagnostic of the electronic character of the complex. In complexes where RNC is primarily a sigma donor ligand, νC≡N shifts to higher energies vs free isocyanide. Thus, for[Co(CN−t−Bu)5]+, νC≡N = 2152, 2120 cm−l.[23] In contrast, for the electron-rich species Fe2(CNEt)9, νC≡N = 2060, 1920 cm−l for the terminal isocyanide ligands (1701, 1652 cm−l for the bridging isocyanides).[28]
^Constable, Edwin C.; Johnson, Brian F.G.; Khan, Fatima K.; Lewis, Jack; Raithby, Paul R.; Mikulcik, Patrizia (1991). "Synthesis and crystal and molecular structure of Os3(CO)9[1,1,1-tris(isocyanomethyl)ethane]: A novel tris-isocyanide ligand capped triosmium cluster".Journal of Organometallic Chemistry.403 (1–2):C15–C18.doi:10.1016/0022-328X(91)83111-G.
^Harvey, P. (2001). "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".Coordination Chemistry Reviews.219–221:17–52.doi:10.1016/S0010-8545(00)00415-X.
^Plummer, Daniel T.;Angelici, Robert J. (1983). "Synthesis and characterization of homoleptic complexes of the chelating bidentate isocyano ligand tert-BuDiNC".Inorganic Chemistry.22 (26):4063–4070.doi:10.1021/ic00168a048.
^abBassett, Jean-Maria; Farrugia, Louis J.; Stone, F. Gordon A. (1980). "Notes. Protonation of Pentakis(t-butyl Isocyanide)iron".Journal of the Chemical Society, Dalton Transactions (9): 1789.doi:10.1039/DT9800001789.
^Fehlhammer, Wolf P.; Fritz, Marcus. (1993). "Emergence of a CNH and cyano complex based organometallic chemistry".Chemical Reviews.93 (3):1243–1280.doi:10.1021/cr00019a016.
^Hahn FE, Jahnke MC (2008). "Heterocyclic Carbenes: Synthesis and Coordination Chemistry".Angewandte Chemie International Edition.47 (17):3122–72.doi:10.1002/anie.200703883.PMID18398856.
^Pombeiro, Armando J.L; Fátima, M.; Guedes Da Silva, C. (2001). "Coordination Chemistry of CNH2, The Simplest Aminocarbyne".Journal of Organometallic Chemistry.617–618:65–69.doi:10.1016/S0022-328X(00)00641-0.
^Vicente J, Abad JA, Fortsch W, Lopez-Saez MJ (2004). "Reactivity of ortho-Palladated Phenol Derivatives with Unsaturated Molecules".Organometallics.23:4414–4429.doi:10.1021/om0496131.
^Barybin, Mikhail V.; Young, Victor G.; Ellis, John E. (2000). "First Paramagnetic Zerovalent Transition Metal Isocyanides. Syntheses, Structural Characterizations, and Magnetic Properties of Novel Low-Valent Isocyanide Complexes of Vanadium1".Journal of the American Chemical Society.122 (19):4678–4691.doi:10.1021/ja000212w.
^abcBarybin, Mikhail V.; Young, Victor G.; Ellis, John E. (1998). "Syntheses and Structural Characterizations of the First 16-, 17-, and 18-Electron Homoleptic Isocyanide Complexes of Vanadium: Hexakis(2,6-dimethylphenyl Isocyanide)vanadium(I, 0, −I)1".Journal of the American Chemical Society.120 (2):429–430.doi:10.1021/ja9729239.
^Bohling, David A.; Mann, Kent R. (1984). "X-ray Structural Characterization of [Cr(CNPh)6]CF3SO3, [Cr(CNPh)6][PF6]2, and [Cr(CNPh)6][SbCl6]3.CH2Cl2. Completion of a Unique Series of Complexes in Which the Metal Attains Four Different Oxidation States While Maintaining Identical Ligation".Inorganic Chemistry.23 (10):1426–1432.doi:10.1021/ic00178a025.
^Dewan, John C.; Mialki, William S.; Walton, Richard A.; Lippard, Stephen J. (1982). "Crystal Structure of the Homoleptic Seven-coordinate Complex Heptakis(tert-butyl Isocyanide)chromium(II) Hexafluorophosphate, [Cr(CN-t-Bu)7](PF6)2".Journal of the American Chemical Society.104:133–136.doi:10.1021/ja00365a025.
^Bohling, David A.; Mann, Kent R. (1984). "X-ray Structural Characterization of [Cr(CNPh)6]CF3SO3, [Cr(CNPh)6](PF6)2, and [Cr(CNPh)6](SbCl6)3.CH2Cl2. Completion of a Unique Series of Complexes in which the Metal Attains Four Different Oxidation States While Maintaining Identical Ligation".Inorganic Chemistry.23 (10):1426–1432.doi:10.1021/ic00178a025.
^J.-M. Bassett, M. Green, J. A. K. Howard, F. G. A. Stone (1978). "Formation of nona(ethyl isocyanide)diiron from penta(ethyl isocyanide)iron and reaction of penta(tert-butyl isocyanide)iron with diphenylacetylene; x-ray crystal structures of nona(ethyl isocyanide)diiron and tris(tert-butyl isocyanide){1,4-bis-(tert-butylimino)-2,3-diphenylbuta-1,3-diene}iron".J. Chem. Soc., Chem. Commun.:1000–1.doi:10.1039/C39780001000.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Barker, Geoffrey K.; Galas, Anita M. R.; Green, Michael; Howard, Judith A. K.; Stone, F. Gordon A.; Turney, Terence W.; Welch, Alan J.; Woodward, Peter (1977). "Synthesis and Reactions of Octakis(t-butyl Isocyanide)dicobalt and Pentakis(t-butyl Isocyanide)ruthenium; X-ray Crystal and Molecular Structures of Co2(ButNC)8 and Ru(Ph3P)(ButNC)4".Journal of the Chemical Society, Chemical Communications (8): 256.doi:10.1039/C39770000256.
^abKing, R. B.; Saran, Mohan Singh (1972). "Isocyanide-metal complexes. I. Reactions of the pentakis(tert-butyl isocyanide)cobalt cation with some tertiary phosphines and arsines".Inorganic Chemistry.11 (9):2112–2119.doi:10.1021/ic50115a025.
^Leach, Patricia A.; Geib, Steven J.; Corella, Joseph A.; Warnock, Garry F.; Cooper, N. John (1994). "Synthesis and Structural Characterization of [Co{CN(2,6-C6H3Me2)]−, the First Transition Metal Isonitrilate".Journal of the American Chemical Society.116 (19):8566–8574.doi:10.1021/ja00098a017.
^Kanters, J. A.; Nijs, H. L. L. M.; Van Der Sluis, P. (1989). "The Structure of the Tetrahydrofuran Solvate of Tetrakis(2,6-diisopropylphenylisocyanide)nickel(II) Diperchlorate".Acta Crystallographica Section C Crystal Structure Communications.45:16–18.doi:10.1107/S0108270188009497.
^Day, V. W.; Day, R. O.; Kristoff, J. S.; Hirsekorn, F. J.; Muetterties, E. L. (1975). "Fluxional, Catalytically Active Metal Cluster, Heptakis(tert-butylisocyanide)tetranickel".Journal of the American Chemical Society.97 (9):2571–2573.doi:10.1021/ja00842a061.
^Stephany, R. W.; de Bie, M. J. A.; Drenth, W. (1974). "A13C-NMR and IR study of isocyanides and some of their complexes".Organic Magnetic Resonance.6 (1):45–47.doi:10.1002/mrc.1270060112.
^Bassett, Jean-Marie; Green, Michael; Howard, Judith A. K.; Stone, F. Gordon A. (1978). "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".Journal of the Chemical Society, Chemical Communications (22): 1000.doi:10.1039/C39780001000.
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