Diiron nonacarbonyl

Diiron nonacarbonyl (referred to as diiron enneacarbonyl in the older literature) is the chemical compound with the formula Fe2(CO)9. This metal carbonyl is an important reagent in organometallic chemistry and of occasional use in organic synthesis. It is a more reactive source of Fe(0) than Fe(CO)5 and less dangerous to handle because it is nonvolatile. One of the strangest properties of this micaceous, orange solid is that it is virtually insoluble in all known solvents.

Synthesis
Photolysis of Fe(CO)5, typically in acetic acid solution, produces Fe2(CO)9 in good yield:
 * 2 Fe(CO)5 →   Fe2(CO)9  + CO

Structure
Fe2(CO)9 consists of a pair of Fe(CO)3 centers linked by three bridging CO ligands. The iron atoms are equivalent and octahedral. Elucidation of the structure of Fe2(CO)9 has proven to be challenging because of its very low solubility, which prevents growth of crystals. The Mößbauer spectrum reveals one quadrupole doublet, consistent with the D3h-symmetric structure.

Reactions
Fe2(CO)9 is primarily employed as a precursor to compounds of the type Fe(CO)4L and Fe(CO)3(diene). Such syntheses are typically conducted in THF solution. In these conversions, it is proposed that small amounts of Fe2(CO)9 dissolve according the following reaction:
 * Fe2(CO)9 →  Fe(CO)5  +  Fe(CO)4(THF)

Cyclobutadieneiron tricarbonyl is prepared using Fe2(CO)9. Fe2(CO)9 has also been employed in the synthesis of cyclopentadienones via a net [2+3]-cycloaddition from dibromoketones.

Safety
Fe2(CO)9 is a source of CO although it is nonvolatile and insoluble.