Diiron nonacarbonyl

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Diiron nonacarbonyl

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Template:Longitem	Fe2C9O9
Molar mass	363.78 g/mol
Appearance	orange crystals
Density	2.08 g/cm3
Melting point	Template:Chembox CalcTemperatures
Template:Longitem	0 D
Template:Longitem	Iron pentacarbonyl Triiron dodecacarbonyl
Template:Longitem	Dimanganese decacarbonyl Dicobalt octacarbonyl

Diiron nonacarbonyl is an organometallic compound with the formula Fe₂(CO)₉. This metal carbonyl is a reagent in organometallic chemistry and of occasional use in organic synthesis. It is a more reactive source of Fe(0) than Fe(CO)₅. This micaceous orange solid is virtually insoluble in all common solvents.

Synthesis and structure

Following the original method, photolysis of an acetic acid solution of Fe(CO)₅ produces Fe₂(CO)₉ in good yield:

2 Fe(CO)₅ → Fe₂(CO)₉ + CO

Fe₂(CO)₉ consists of a pair of Fe(CO)₃ centers linked by three bridging CO ligands. Although older textbooks show an Fe-Fe bond consistent with the 18 electron rule (8 valence electrons from Fe, two each from the terminal carbonyls, one each from the bridging carbonyls and one from the other Fe atom in the metal-metal bond), theoretical analyses have consistently indicated the absence of a direct Fe-Fe bond: this latter model proposes an Fe-C-Fe three-center-two-electron "banana bond" for one of the bridging carbonyls. The minor isomer has been crystallized together with C₆₀. The iron atoms are equivalent and octahedral molecular geometry. Elucidation of the structure of Fe₂(CO)₉ proved to be challenging because its low solubility inhibits growth of crystals. The Mößbauer spectrum reveals one quadrupole doublet, consistent with the D_3h-symmetric structure.

Reactions

Template:Chem2 is a precursor to compounds of the type Fe(CO)₄L and Fe(CO)₃(diene). Such syntheses are typically conducted as tetrahydrofuran (THF) slurries. In these conversions, it is proposed that small amounts of Fe₂(CO)₉ dissolve according to the following reaction:

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Alkene complexes arise by direct reaction of a suspension in warm benzene:

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In related reaction (benzylideneacetone)iron tricarbonyl, with the formula Template:Chem2 is prepared by the reaction of diiron nonacarbonyl with benzylideneacetone. This complex is a source of the Fe(CO)₃ fragment. Oxidative addition of allyl bromide to diiron nonacarbonyl gives the allyl iron(II) derivative:

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Cyclobutadieneiron tricarbonyl is prepared similarly using 3,4-dichlorocyclobutene, as described in the following idealized equation:

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Template:Chem2 has also been employed in the synthesis of cyclopentadienones via a net [2+3]-cycloaddition from dibromoketones, known as the Noyori [3+2] reaction.

Low temperature (15 K) UV/vis photolysis of Fe₂(CO)₉ yields the unsaturated complex Fe₂(CO)₈, which exists as both CO-bridged and unbridged isomers.

Safety

Metal carbonyls are typically treated as if they are highly toxic.