Metal carbonyl

Metal carbonyls are coordination complexes of transition metals with carbon monoxide.^[1] These complexes may be homoleptic, i.e. contain 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 for many compounds, e.g. 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.

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Bonding

Carbon monoxide bonds to transition metals using "synergic pi* 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.

Compounds

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 (CH₃C₅H₄)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 carbons 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

• Group 4 elements as dianions resemble neutral group 6 derivatives: [Ti(CO)₆]^2-.^[2]
• Group 5 elements as monoanions resemble again neutral group 6 derivatives: [V(CO)₆]^-.
• Group 7 elements as monoanions resemble neutral group 8 derivatives: [M(CO)₅]^- (M = Mn, Tc, Re).
• Group 8 elements as dianaions resemble neutral group 10 derivatives: [M(CO)₄]^2- (M = Fe, Ru, Os). Condensed derivatives are also known.
• Group 9 elements as monoanions resemble neutral group 10 metal carbonyl. [Co(CO)₄]^- is the best studied member.

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).^[3]

Synthesis

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 in the laboratory by abstraction of CO from solvents such as dimethylformamide (DMF) and methoxyethanol. Typical is the synthesis of IrCl(CO)(PPh₃)₂ from the reaction of iridium(III) chloride and triphenylphosphine in boiling DMF solution.

Metal carbonyl hydrides

Metal carbonyls are relatively distinctive in forming complexes is 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.^[4]

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.

Thiocarbonyls

Complexes containing CS are known but are uncommon.^[5] 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 closely parallel those of the metal carbonyls. In contrast to PF₃, alkyl- and arylphosphines can be substituted for CO in metal carbonyls but homoleptic tertiary phosphine complexes that are analogous to neutral and anionic carbonyls are rare.

Complexes of Isocyanides

Isocyanides also form extensive families of complexes that are related to the metal carbonyls. Typical isocyanide ligands are CH₃NC and t-butylisocyanide ("t-BuNC") (CH₃)₃CNC, A special case is CF₃NC, an unstable molecule that forms stable complexes whose behavior closely parallels that of the metal carbonyls.

IR spectroscopy

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

In addition to their frequency, the number of the ν_CO bands is diagnostic of structure of the complex. Octahedral complexes, e.g. Cr(CO)₆, exhibits only a single ν_CO band in its IR spectrum. Spectra for complexes of lower symmetry are more complex. The IR spectrum of Fe₂(CO)₉ displays CO bands at 2082, 2019, 1829 cm^-1. For Fe(CO)₅, ν_CO = 2022, 2000 cm^-1.

Bonding modes in clusters

The carbonyl ligand is versatile and displays a range of bonding modes in metal carbonyl cluster chemistry.^[6] The most frequently encountered mode is terminal (see above), but bridging between two (μ₂) or three (μ₃) metals is also common. The increased π-bonding due to back-donation from more metal centers results in further weakening of the C-O bond. Much less common are bonding modes in which both C and O bond to the metal, e.g. μ₃-η².

Occurrence in nature

The hydrogenase enzymes contain CO bound to iron, apparently the CO stabilizes low oxidation states which facilitates the binding of hydrogen.^[7] Certain metal carbonyls have been observed in trace amounts in landfills, where the reducing environment is compatible with their formation.^[8]

History

It has long been known that transition metals form complexes with carbon monoxide as a ligand. Ludwig Mond prepared Ni(CO)₄ in the 1880s, which eventually led to the synthesis of many analogues, led primarily by Walter Hieber who prepared the first metal hydride, H₂Fe(CO)₄.

Properties

Metal carbonyls generally have poor misciblity with water.

References

1. ^ C. Elschenbroich, A. Salzer ”Organometallics : A Concise Introduction” (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7.
2. ^ Ellis, J. E. "Metal Carbonyl Anions: from [Fe(CO)₄]^2- to [Hf(CO)₆]^2- and Beyond" Organometallics, 2003, volume 22, 3322-3338.
3. ^ Finze, M.; Bernhardt, E.; Willner, H.; Lehmann, C. W.; Aubke, F., "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)", Inorganic Chemistry, 2005, volume 44, pages 4206-4214.
4. ^ Pearson, Ralph G. “The Transition-Metal-Hydrogen Bond” Chemical Reviews 1995, volume 95, 41.
5. ^ Hill, A. F.; Wilton-Ely, J. D. E. T. "Chlorothiocarbonyl-bis(triphenylphosphine) iridium(I) [IrCl(CS)(PPh₃)₂], Inorganic Syntheses, 2002, volume 33, pages 244-245.
6. ^ P. J. Dyson and J. S. McIndoe, Transition Metal Carbonyl Cluster Chemistry, Gordon & Breach: Amsterdam (2000). ISBN 9056992899.
7. ^ Bioorganometallics: Biomolecules, Labeling, Medicine; Jaouen, G., Ed. Wiley-VCH: Weinheim, 2006.3-527-30990-X.
8. ^ Feldmann, J. “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, 1999, 1, page 33-37. DOI: 10.1039/a807277i.

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