Iron pentacarbonyl

Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula Fe(CO)₅. Under standard conditions Fe(CO)₅ is a free-flowing, straw-colored liquid with a pungent odour. This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis.^[1] Fe(CO)₅ is prepared by the reaction of fine iron particles with carbon monoxide. Fe(CO)₅ is inexpensively purchased.

Iron pentacarbonyl is one of the homoleptic metal carbonyls; i.e. metal complexes bonded only to CO ligands. Other examples include octahedral Cr(CO)₆ and tetrahedral Ni(CO)₄. Most metal carbonyls have 18 valence electrons, and Fe(CO)₅ fits this pattern with 8 valence electrons on Fe and five pairs of electrons provided by the CO ligands. Reflecting its symmetrical structure and charge neutrality, Fe(CO)₅ is volatile; it is one of the most frequently encountered liquid metal complexes. Fe(CO)₅ adopts a trigonal bipyramidal structure with the Fe atom surrounded by five CO ligands: three in equatorial positions and two axially bound. The Fe-C-O linkages are each linear.

Fe(CO)₅ is the archetypal fluxional molecule due to the rapid interchange of the axial and equatorial CO groups via the Berry mechanism on the NMR timescale. Consequently, the¹³C 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.

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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 Fe₂(CO)₉, a yellow-orange solid, also described by Mond. When heated, Fe(CO)₅ converts to small amounts of the metal cluster 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 isocyanides, tertiary phosphines and arsines, and alkenes. Usually these ligands displace only one or two CO ligands, but certain acceptor ligands such as PF₃ and isocyanides can proceed to tetra- and pentasubstitution. These reactions are often induced with a catalyst or light.^[3] Illustrative is the synthesis of the bis(triphenylphosphine) complex Fe(CO)₃(P(C₆H₅)₃)₂.^[4] This transformation can be accomplished photochemically, but it is also induced by the addition of NaOH or 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 halogens gives the ferrous halides Fe(CO)₄X₂ for X = I, Br, Cl. These species, upon heating lose CO to give the ferrous halides, such as iron(II) chloride.

Reduction of Fe(CO)₅ with Na gives Na₂Fe(CO)₄, "tetracarbonylferrate" also called Collman's reagent. The dianion is isoelectronic with Ni(CO)₄ but highly nucleophilic.^[5]

Acid-base reactions

Fe(CO)₅ is not readily protonated, but it is attacked by hydroxide. Treatment of Fe(CO)₅ with aqueous base produces [HFe(CO)₄]^-, the 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 norbornadiene and 1,3-butadiene. One of the more historically significant derivatives is cyclobutadieneiron tricarbonyl (C₄H₄)Fe(CO)₃, where C₄H₄ is the otherwise unstable cyclobutadiene.^[6] Receiving the greatest attention are complexes of the cyclohexadienes, the parent organic 1,4-dienes being available through the Birch reductions. 1,4-Dienes isomerize to the 1,3-dienes upon complexation.^[7]

Fe(CO)₅ reacts in dicyclopentadiene to form [Fe(C₅H₅)(CO)₂]₂, cyclopentadienyliron dicarbonyl dimer. This compound, called "Fp dimer" can be considered a hybrid of 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 tetraethyllead. Two more modern alternative fuel additives are ferrocene and methylcyclopentadienyl manganese tricarbonyl. Fe(CO)₅ is used in the production of "carbonyl iron", a finely divided form of Fe, a material used in magnetic cores of high-frequency coils for radios and televisions and for manufacture of the active ingredients of some radar absorbent materials (e.g. iron ball paint). It is famous as a chemical precursor for the synthesis of various iron-based 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. ^ Birch, A. J.; Chamberlain, K. B.. "Tricarbonyl[(2,3,4,5-eta)-2,4-Cyclohexadien-1-one]ison and Tricarbonyl[(1,2,3,4,5-eta)-2-Methoxy-2,4-Cyclohexadien-1-yl]Iron(1+) Hexafluorophosphate(1-) from Anisole". Org. Synth.; Coll. Vol. 6: 996. 
8. ^ Lask, G. & Wagner, H. Gg., " ", Eighth International Symposium on Combustion: 432-438