Everything about Iron Pentacarbonyl totally explained
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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; for example
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's 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 isn't 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 PF
3 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(C
6H
5)
3)
2. This transformation can be accomplished photochemically, but it's 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)
4X
2 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 isn't readily
protonated, but it's attacked by
hydroxide. Treatment of Fe(CO)
5 with aqueous base produces [HFe(CO)
4]
-, the
oxidation of which gives Fe
3(CO)
12. Acidification of solutions of [HFe(CO)
4]
- gives H
2Fe(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 (C
4H
4)Fe(CO)
3, where C
4H
4 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(C
5H
5)(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 (for example
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 hasn't 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.
Further Information
Get more info on 'Iron Pentacarbonyl'.
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