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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]
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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.
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. μ3-η2.
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:
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.
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.
Large anionic clusters of Ni, Pd, and Pt are also well known.
| 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]
Many analogues of CO ligands are known to form homoleptic and mixed ligand complexes.
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 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:
Complexes of CSe and CTe are very rare.
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.
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.
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