Diiron nonacarbonyl is an organometallic compound with the formula Fe<sub>2</sub>(CO)<sub>9</sub>. 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)<sub>5</sub>. This micaceous orange solid is virtually insoluble in all common solvents.
Following the original method, photolysis of an acetic acid solution of Fe(CO)<sub>5</sub> produces Fe<sub>2</sub>(CO)<sub>9</sub> in good yield:
Fe<sub>2</sub>(CO)<sub>9</sub> consists of a pair of Fe(CO)<sub>3</sub> 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 C<sub>60</sub>. The iron atoms are equivalent and octahedral molecular geometry. Elucidation of the structure of Fe<sub>2</sub>(CO)<sub>9</sub> proved to be challenging because its low solubility inhibits growth of crystals. The MöÃÂbauer spectrum reveals one quadrupole doublet, consistent with the D<sub>3h</sub>-symmetric structure.
is a precursor to compounds of the type Fe(CO)<sub>4</sub>L and Fe(CO)<sub>3</sub>(diene). Such syntheses are typically conducted as tetrahydrofuran (THF) slurries. In these conversions, it is proposed that small amounts of Fe<sub>2</sub>(CO)<sub>9</sub> dissolve according to the following reaction:
Alkene complexes arise by direct reaction of a suspension in warm benzene:
In related reaction (benzylideneacetone)iron tricarbonyl, with the formula is prepared by the reaction of diiron nonacarbonyl with benzylideneacetone. This complex is a source of the Fe(CO)<sub>3</sub> fragment. Oxidative addition of allyl bromide to diiron nonacarbonyl gives the allyl iron(II) derivative:
Cyclobutadieneiron tricarbonyl is prepared similarly using 3,4-dichlorocyclobutene, as described in the following idealized equation:
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 Fe<sub>2</sub>(CO)<sub>9</sub> yields the unsaturated complex Fe<sub>2</sub>(CO)<sub>8</sub>, which exists as both CO-bridged and unbridged isomers.
Metal carbonyls are typically treated as if they are highly toxic.