Iron pentacarbonyl

Iron pentacarbonyl
IUPAC name	pentacarbonyliron
Other names	Pentacarbonyl iron Iron carbonyl
Identifiers
CAS number	13463-40-6 Y
PubChem	26040
UN number	1994
RTECS number	NO4900000
Properties
Molecular formula	Fe(CO)5
Molar mass	195.90 g/mol
Appearance	straw-yellow liquid
Density	1.45 g/cm3
Melting point	−20 °C
Boiling point	103 °C
Solubility in water	Insoluble
Solubility in organic solvents	Soluble
Structure
Coordination geometry	trigonal bipyramidal
Molecular shape	trigonal bipyramidal
Dipole moment	0 D
Hazards
MSDS	ICSC 0168
Main hazards	Very toxic, highly flammable
NFPA 704	3 4 1
Flash point	−15 °C
Autoignition temperature	50 °C
Explosive limits	3.7–12.5%
Related compounds
Other cations	Triruthenium dodecacarbonyl Triosmium dodecacarbonyl
Related iron carbonyls	Diiron nonacarbonyl Triiron dodecacarbonyl
Related compounds	Dimanganese decacarbonyl Dicobalt octacarbonyl Nickel tetracarbonyl
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.

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 iron carbonyl exhibits distinct reactivity.

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 almost 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. It is, however, considerably less toxic than nickel tetracarbonyl.

References