Iron pentacarbonyl

Iron pentacarbonyl, also known as iron carbonyl, is the carbon monoxide. Fe(CO)₅ is inexpensively purchased.

Iron pentacarbonyl is one of the ligands: three in equatorial positions and two axially bound. The Fe-C-O linkages are each linear.

Fe(CO)₅ is the archetypal NMR spectrum exhibits only one signal due to the rapid interchange between nonequivalent CO sites.

Iron carbonyl is sometimes confused with carbonyl iron, a high-purity metal prepared by decomposition of iron pentacarbonyl.

Synthesis and other iron carbonyls

The compound was described in a journal by Mond and Langer in 1891 as "a somewhat viscous liquid of a pale-yellow colour.".^[2] Samples were prepared by treatment of finely divided, oxide-free iron powder with carbon monoxide at room temperature.

Photolysis of Fe(CO)₅ produces Fe₃(CO)₁₂, a green solid, although simple thermolysis is not a useful synthesis (see below). Each of these three iron carbonyls exhibits distinct reactivity - after all, they are three different compounds.

Key reactions

CO substitution reactions

Thousands of compounds are derived from Fe(CO)₅. Substitution of CO by Lewis bases, L, to give derivatives Fe(CO)_5-xL_x. Common Lewis bases include NaBH₄. The catalyst attacks a CO ligand, which labilizes another CO ligand toward substitution. The electrophilicity of Fe(CO)₄L is less than that of Fe(CO)₅, so the nucleophilic catalyst, disengages and attacks another molecule of Fe(CO)₅.

Oxidation and reduction

Most metal carbonyls can be halogenated. Thus, treatment of Fe(CO)₅ with iron(II) chloride.

Reduction of Fe(CO)₅ with Na gives nucleophilic.^[5]

Acid-base reactions

Fe(CO)₅ is not readily protonated, but it is attacked by oxidation of which gives Fe₃(CO)₁₂. Acidification of solutions of [HFe(CO)₄]^- gives H₂Fe(CO)₄, the first metal hydride ever reported.

Diene adducts

Dienes react with Fe(CO)₅ to give (diene)Fe(CO)₃, wherein two CO ligands have been replaced by two olefins. Many dienes undergo this reaction, notably Birch reductions. 1,4-Dienes isomerize to the 1,3-dienes upon complexation.^[7]

Fe(CO)₅ reacts in ferrocene and Fe(CO)₅, although in terms of its reactivity, it resembles neither.

Other uses

In Europe, iron pentacarbonyl was once used as an anti-knock agent in petrol in place of nanoparticles.

Iron pentacarbonyl has been found to be a strong flame speed inhibitor in oxygen based flames.^[8] Few hundred ppm of iron pentacarbonyl are known to reduce the flame speed of stoichiometric methane-air flame by al most 50%. However due to its toxic nature it has not been used widely as flame retardant.

Toxicity and hazards

Fe(CO)₅ is toxic, which is of concern because of its volatility (vapour pressure: 21 mmHg at 20 °C). If inhaled, iron pentacarbonyl may cause lung irritation, toxic pneumonitis, or pulmonary edema. Like other metal carbonyls, Fe(CO)₅ is flammable.

References

1. ^ Samson, S. ; Stephenson, G. R. "Pentacarbonyliron" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
2. ^ Mond, L.; Langer, C. (1891). "On iron carbonyls". J. Chem. Soc., Trans. 59: 1090 - 1093. doi:10.1039/CT8915901090.
3. ^ Therien, M. J. and Trogler, W. C., "Bis(phosphine) derivatives of iron pentacarbonyl and tetracarbonyl(tri-tert-butylphosphine)iron(0)", Inorganic Syntheses, 1990, 28, 173-9 (photochemical reactions).
4. ^ Keiter, R. L.; Keiter, E. A.; Boecker, C. A.; Miller, D. R. and Hecker, K. H., "Tricarbonylbis(phosphine)iron(0) complexes", Inorganic Syntheses, 1997, 31, 210-214.
5. ^ Finke, R. G.; Sorrell, T. N.. "Nucleophilic Acylation with Disodium Tetracarbonylferrate: Methyl 7-Oxoheptanoate and Methyl 7-oxooctonoate". Org. Synth.; Coll. Vol. 6: 807. 
6. ^ Pettit, R.; Henery, J.. "Cyclobutadieneiron Tricarbonyl". Org. Synth.; Coll. Vol. 6: 310. 
7. ^ Org. Synth.; Coll. Vol. 6: 996. 
8. ^ Lask, G. & Wagner, H. Gg., " ", Eighth International Symposium on Combustion: 432-438