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Iron pentacarbonyl
An iron atom with five CO ligands

Metal carbonyls are coordination complexes of transition metals with carbon monoxide. These complexes may be homoleptic, that is containing only CO ligands, such as nickel carbonyl (Ni(CO)4), but more commonly metal carbonyls contain a mix of ligands, such as Re(CO)3(2,2'-bipyridine)Cl. Carbon monoxide is an important building block for the synthesis of many compounds, for example hydroformylation, and metal carbonyl catalysts are central to its utilization. Metal carbonyls are toxic, in part because of their ability to carbonylate hemoglobin to give carboxyhemoglobin, which will not bind O2.[1]


Structure and properties

The HOMO of CO is a σ MO
The LUMO of CO is a π* antibonding MO

Metal carbonyls generally have poor solubility with water.


Carbon monoxide bonds to transition metals using "synergic π* back-bonding." The bonding has three components, giving rise to a partial triple bond. A sigma bond arises from overlap of nonbonding electron pair on carbon with a blend of d-, s-, and p-orbitals on the metal. A pair of π bonds arises from overlap of filled d-orbitals on the metal with a pair of π-antibonding orbitals projecting from the carbon of the CO. The latter kind of binding requires that the metal have d-electrons, and that the metal is in a relatively low oxidation state (<+2). The π-bonding has the effect of weakening the carbon-oxygen bond compared with free carbon monoxide. Because of the multiple bond character of the M-CO linkage, the distance between the metal and carbon is relatively short, often < 1.8 Â, about 0.2 Â shorter than a metal-alkyl bond.


Bonding modes in clusters

The carbonyl ligand engages in a range of bonding modes in metal carbonyl cluster chemistry.[1][2] Most frequently CO binds in the familiar terminal mode (see above), but CO is often bridges between two (μ2) or three (μ3) metals. Much less common are bonding modes in which both C and O bond to the metal, e.g. μ32.


The increased π-bonding due to back-donation from multiple metal centers results in further weakening of the C-O bond.


The most important technique for characterizing metal carbonyls is infra-red spectroscopy. The C-O vibration, typically called νCO, occurs at 2143 cm-1 for CO gas. The positions of the νCO band(s) for the metal carbonyls is inversely correlated with the strength of the pi-bonding between the metal and the carbon:

Compound νCO (cm-1)
CO 2143
Ti(CO)6-2 1748
V(CO)6-1 1859
Cr(CO)6 2000
Mn(CO)6+ 2100
Fe(CO)62+ 2204
Fe(CO)5 2022, 2000

In addition to their frequency, the number of the νCO bands is diagnostic of structure of the complex. Octahedral complexes, e.g. Cr(CO)6, exhibits only a single νCO band in its IR spectrum. Spectra for complexes of lower symmetry are more complex. For example, the IR spectrum of Fe2(CO)9 displays CO bands at 2082, 2019, 1829 cm-1.

In cluster carbonyls, νCO is a sensitive probe for the CO coordination geometry. For bridging (μ2) ligands νCO is usually shifted by 100-200 cm-1 to lower wavenumbers compared to the signatures of μ1-CO. Bands for face capping (μ3) CO ligands appear at even lower energies. Typical values for rhodium cluster carbonyls are:[3]

carbonyl νCO, µ1 (cm-1) νCO, µ2 (cm-1) νCO, µ3 (cm-1)
Rh2(CO)8 2060, 2084 1846, 1862
Rh4(CO)12 2044, 2070, 2074 1886
Rh6(CO)16 2045, 2075 1819


Although nickel carbonyl and iron pentacarbonyl form upon treatment of the metals with carbon monoxide, most metal carbonyls are prepared less directly. The other homoleptic carbonyls are usually made by "reductive carbonylation" of metal salts or metal oxides under a high pressure of carbon monoxide in autoclave:

Re2O7 + 17 CO → Re2(CO)10 + 7 CO2

Once prepared, these homoleptic carbonyls undergo extensive substitution and redox reactions.

Mixed ligand carbonyls of ruthenium, osmium, rhodium, and iridium are often generated by abstraction of CO from solvents such as dimethylformamide (DMF) and 2-methoxyethanol. Typical is the synthesis of IrCl(CO)(PPh3)2 from the reaction of iridium(III) chloride and triphenylphosphine in boiling DMF solution.

Occurrence in nature

The hydrogenase enzymes contain CO bound to iron, apparently the CO stabilizes low oxidation states which facilitates the binding of hydrogen.[4] Certain metal carbonyls have been observed in trace amounts in landfills, where the reducing environment is compatible with their formation.[5]


Most metal carbonyl complexes contain a mixture of ligands. Examples include the historically important IrCl(CO)(P(C6H5)3) 2 and the anti-knock agent (CH3C 5H4)Mn(CO)3. The parent compounds for many of these mixed ligand complexes are the binary carbonyls, i.e. species of the formula [M(CO)n]z, many of which are commercially available. The formula of many metal carbonyls can be inferred from the 18 electron rule.

Charge-neutral binary metal carbonyls

  • Group 4 elements with 4 valence electrons are rare, but substituted derivatives of Ti(CO)7 are known.
  • Group 5 elements with 5 valence electrons, again are subject to steric effects that prevent the formation of M-M bonded species such as V2(CO)12, which is unknown. The 17 VE V(CO)6 is however well known.
  • Group 6 elements with 6 valence electrons form metal carbonyls Cr(CO)6, Mo(CO)6, and W(CO)6 (6 + 6x2 = 18 electrons).
  • Group 7 elements with 7 valence electrons form metal carbonyl dimers Mn2(CO)10, Tc2(CO)10, and Re2(CO)10 (7 + 1 + 5x2 = 18 electrons).
  • Group 8 elements with 8 valence electrons form metal carbonyls Fe(CO)5,Ru(CO)5 and Os(CO)5 (8 + 5x2 = 18 electrons). The heavier two members are unstable, tending to decarbonylate to give Ru3(CO)12, and Os3(CO)12. The two other principal iron carbonyls are Fe3(CO)12 and Fe2(CO)9.
  • Group 9 elements with 9 valence electrons and are expected to form metal carbonyl dimers M2(CO)8. In fact the cobalt derivative of this octacarbonyl is the only stable member, but all three tetramers are well known: Co4(CO)12, Rh4(CO)12, and Ir4(CO)12 (9 + 3 + 3x2 = 18 electrons). Co2(CO)8 unlike the majority of the other 18 VE transition metal carbonyls is sensitive to oxygen.
  • Group 10 elements with 10 valence electrons form metal carbonyls Ni(CO)4 (10 + 4x2 = 18 electrons). Curiously Pd(CO)4 and Pt(CO)4 are not stable.

Anionic binary metal carbonyls

Large anionic clusters of Ni, Pd, and Pt are also well known.

Cationic binary metal carbonyls

  • Group 7 elements as monocations resemble neutral group 6 derivative [M(CO)6]+ (M = Mn, Tc, Re).
  • Group 8 elements as dications also resemble neutral group 6 derivatives [M(CO)6]2+ (M = Fe, Ru, Os).[7]

Metal carbonyl hydrides

Metal Carbonyl hydride pKa
HCo(CO)4 "strong"
HCo(CO)3(P(OPh)3) 5.0
HCo(CO)3(PPh3) 7.0
HMn(CO)5 7.1
H2Fe(CO)4 4.4, 14
[HCo(dmgH)2PBu3 10.5

Metal carbonyls are relatively distinctive in forming complexes with negative oxidation states. Examples include the anions discussed above. These anions can be protonated to give the corresponding metal carbonyl hydrides. The neutral metal carbonyl hydrides are often volatile and can be quite acidic.[8]

Related compounds

Many analogues of CO ligands are known to form homoleptic and mixed ligand complexes.

Complexes of nitrosyls

Metal nitrosyls, featuring NO as a ligand are numerous, although homoleptic derivatives are not. Relative to CO, NO is a stronger acceptor and isocyanides are better donors. Well known nitrosyl carbonyls include CoNO(CO)3 and Fe(NO)2(CO)2.[9]

Complexes of thiocarbonyls

Complexes containing CS are known but are uncommon.[10][11] The rarity of such complexes is attributable in part to the fact that the obvious source material, carbon monosulfide, is unstable. Thus, the synthesis of thiocarbonyl complexes requires more elaborate routes, such as the reaction of disodium tetracarbonylferrate with thiophosgene:

Na2Fe(CO)4 + CSCl2 → Fe(CO)4CS + 2 NaCl

Complexes of CSe and CTe are very rare.

Complexes of PF3

Complexes of PF3 often parallel those of the metal carbonyls. In contrast to PF3, alkyl- and arylphosphines can be substituted for CO in metal carbonyls, but rarely give homoleptic complexes analogous to the carbonyls.

Complexes of isocyanides

Isocyanides also form extensive families of complexes that are related to the metal carbonyls. Typical isocyanide ligands are MeNC and t-butylisocyanide ((Me3CNC). A special case is CF3NC, an unstable molecule that forms stable complexes whose behavior closely parallels that of the metal carbonyls.


Ludwig Mond prepared Ni(CO)4 in the 1880s, which eventually led to the synthesis of many analogues, primarily by Walter Hieber who prepared the first metal hydride, H2Fe(CO)4 and the first metal carbonyl halide Fe(CO)4I2. Hieber also established the nuclearity of first metal carbonyl cluster, Fe3(CO)12. The economic benefits of metal-catalysed carbonylations, e.g. Reppe Chemistry and hydroformylation, led to growth of the area.

See also

External links


  1. ^ a b Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6
  2. ^ P. J. Dyson and J. S. McIndoe, Transition Metal Carbonyl Cluster Chemistry, Gordon & Breach: Amsterdam (2000). ISBN 9056992899.
  3. ^ A.D. Allian, Y. Wang, M. Saeys, G.M. Kuramshina, M. Garland (2006). "The combination of deconvolution and density functional theory for the mid-infrared vibrational spectra of stable and unstable rhodium carbonyl clusters". Vibrational Spectroscopy 41: 101–111. doi:10.1016/j.vibspec.2006.01.013.  
  4. ^ Bioorganometallics: Biomolecules, Labeling, Medicine; Jaouen, G., Ed. Wiley-VCH: Weinheim, 2006.3-527-30990-X.
  5. ^ Feldmann, J. (1999). "Determination of Ni(CO)4, Fe(CO)5, Mo(CO)6, and W(CO)6 in sewage gas by using cryotrapping gas chromatography inductively coupled plasma mass spectrometry". Journal of Environmental Monitoring 1: 33–37. doi:10.1039/a807277i.  
  6. ^ Ellis, J. E. (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2- to [Hf(CO)6]2- and Beyond". Organometallics 22: 3322–3338. doi:10.1021/om030105l.  
  7. ^ Finze, M.; Bernhardt, E.; Willner, H.; Lehmann, C. W.; Aubke, F. (2005). "Homoleptic, σ-Bonded Octahedral Superelectrophilic Metal Carbonyl Cations of Iron(II), Ruthenium(II), and Osmium(II). Part 2: Syntheses and Characterizations of [M(CO)6][BF4]2 (M = Fe, Ru, Os)". Inorg. Chem. 44: 4206–4214. doi:10.1021/ic0482483.  
  8. ^ Pearson, Ralph G. (1995). "The Transition-Metal-Hydrogen Bond". Chem. Rev. 95: 41. doi:10.1021/cr00065a002.  
  9. ^ Hayton, T. W.; Legzdins, P. and Sharp, W. B., "Coordination and Organometallic Chemistry of Metal-NO Complexes", Chemical Reviews, 2002, volume 102, 935-991.doi:10.1021/cr000074t
  10. ^ Petz, W., "40 years of transition-metal thiocarbonyl chemistry and the related CSe and CTe compounds", Coordination Chemistry Reviews, 2008, 252, 1689-1733.doi:10.1016/j.ccr.2007.12.011.
  11. ^ Hill, A. F.; Wilton-Ely, J. D. E. T. (2002). "Chlorothiocarbonyl-bis(triphenylphosphine) iridium(I) [IrCl(CS)(PPh3)2]". Inorg. Synth. 33: 244–245. doi:10.1002/0471224502.ch4.  


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