Iron pentacarbonyl

Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula 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. 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, the 13C 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.". 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.

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-xLx. 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. Illustrative is the synthesis of the bis(triphenylphosphine) complex Fe(CO)3(P(C6H5)3)2. 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.

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. 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.

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. 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.