Iron pentacarbonyl


Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula. Under standard conditions Fe5 is a free-flowing, straw-colored liquid with a pungent odour. Older samples appear darker. This compound is a common precursor to diverse iron compounds, including many that are useful in small scale organic synthesis.

Properties

Iron pentacarbonyl is a homoleptic metal carbonyl, where carbon monoxide is the only ligand complexed with a metal. Other examples include octahedral chromium hexacarbonyl and tetrahedral nickel carbonyl. Most metal carbonyls have 18 valence electrons, and Fe5 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, Fe5 is volatile; it is one of the most frequently encountered liquid metal complexes. Fe5 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.
Fe5 exhibits a relatively low rate of interchange between the axial and equatorial CO groups via the Berry mechanism. It is characterized by two intense νCO bands in the IR spectrum at 2034 and 2014 cm−1.

Synthesis and other iron carbonyls

Fe5 is produced by the reaction of fine iron particles with carbon monoxide. 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.
Industrial synthesis of the compound requires relatively high temperatures and pressures as well as specialized, chemically resistant equipment. Preparation of the compound at the laboratory scale avoids these complications by using an iodide intermediate:
  1. FeI2 + 4 CO → Fe(CO)4I2
  2. 5 Fe4I2 + 10 Cu → 10 CuI + 4 Fe5 + Fe

Industrial production and use

The industrial production of this compound is somewhat similar to the Mond process in that the metal is treated with carbon monoxide to give a volatile gas. In the case of iron pentacarbonyl, the reaction is more sluggish. It is necessary to use iron sponge as the starting material, and harsher reaction conditions of 5–30 MPa of carbon monoxide and 150–200 °C. Similar to the Mond process, sulfur acts as a catalyst. The crude iron pentacarbonyl is purified by distillation. Ullmann's Encyclopedia of Industrial Chemistry reports that there are only three plants manufacturing pentacarbonyliron; BASF in Germany and American Carbonyl in Alabama have capacities of 9000 and 1500–2000 tonnes/year respectively.
Most iron pentacarbonyl produced is decomposed on site to give pure carbonyl iron in analogy to carbonyl nickel. Some iron pentacarbonyl is burned to give pure iron oxide. Other uses of pentacarbonyliron are small in comparison.

Reactions

Many compounds are derived from Fe5 by substitution of CO by Lewis bases, L, to give derivatives Fe5−xLx. Illustrative is the synthesis of the bis(triphenylphosphine)iron tricarbonyl complex. 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. In addition to the photochemical route, substitution can also induced by NaOH or NaBH4. The catalyst attacks a CO ligand, which labilizes another CO ligand toward substitution. The electrophilicity of Fe4L is less than that of Fe5, so the nucleophilic catalyst, disengages and attacks another molecule of Fe5.

Photochemical decarbonylation

Irradiation of Fe5 with UV produces Fe4, which captures a variety of ligands to give adducts. Specifically, excitation of the metal-to-CO charge-transfer band induces CO photolysis, generating singlet and triplet coordinatively-unsaturated Fe4 with high quantum yield. Prolonged irradiation in gas phase may proceed to further CO detach until atomic Fe formation.
In the absence of trapping substrates, Fe2(CO)9 is produced.

Oxidation and reduction

Most metal carbonyls can be halogenated. Thus, treatment of Fe5 with iodine gives iron tetracarbonyl diiodide:
Reduction of Fe5 with Na gives Na2Fe(CO)4, "tetracarbonylferrate" also called Collman's reagent. The dianion is isoelectronic with Ni4 but highly nucleophilic.

Acid-base reactions

Fe5 is not readily protonated, but it is attacked by hydroxide. Treatment of Fe5 with aqueous base produces Potassium tetracarbonyliron hydride|−, via the metallacarboxylate intermediate. The oxidation of this monoanion gives triiron dodecacarbonyl, Fe312. Acidification of solutions of − gives iron tetracarbonyl dihydride, H2Fe4.
Likewise, Fe5 reacts with HgSO4 to give the polymer n.

Diene adducts

Dienes react with Fe5 to give Fe3, wherein two CO ligands have been replaced by two olefins. Many dienes undergo this reaction, e.g. norbornadiene and 1,3-butadiene. One of the more historically significant derivatives is cyclobutadieneiron tricarbonyl Fe3, where C4H4 is the otherwise unstable cyclobutadiene.
Fe5 reacts in dicyclopentadiene to form 2, cyclopentadienyliron dicarbonyl dimer. This compound, called "Fp dimer" can be considered a hybrid of ferrocene and Fe5, 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; it was produced by IG Farben and commercially marketed under the trade names, “Motolin” and “Monopolin”. Two more modern alternative fuel additives are ferrocene and methylcyclopentadienyl manganese tricarbonyl. Fe5 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. 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. A few hundred ppm of iron pentacarbonyl are known to reduce the flame speed of stoichiometric methane–air flame by almost 50%. However due to its toxic nature it has not been used widely as a flame retardant.

Toxicity and hazards

Fe5 is toxic, which is of concern because of its volatility. If inhaled, iron pentacarbonyl may cause lung irritation, toxic pneumonitis, or pulmonary edema. Like other metal carbonyls, Fe5 is flammable. It is, however, considerably less toxic than nickel tetracarbonyl.
The National Institute for Occupational Safety and Health has set a recommended exposure limit for iron pentacarbonyl at 0.1 ppm over an eight-hour time-weighted average, and a short-term exposure limit at 0.2 ppm.