(Diene)iron tricarbonyl

In organometallic chemistry, (diene)iron tricarbonyl describes a diverse family of related coordination complexes consisting of a diene ligand coordinated to a Fe(CO)3 center. Often the diene is conjugated, e.g., butadiene, but the family includes nonconjugated dienes as well. The compounds are yellow, air-stable, often low-melting, and soluble in hydrocarbon solvents. The motif is so robust that even unstable dienes form easily characterized derivatives, such as norbornadienone and cyclobutadiene.

Scope
The inventory of complexes is large.

Preparation and uses
Many of diene complexes were originally prepared by reaction of iron pentacarbonyl with the diene under UV-radiation. Often yields are modest because the complexes, which are often liquids, volatilize during workup. Some derivatives derivatives are prepared displacement of bda from (benzylideneacetone)iron tricarbonyl (Fe(bda)(CO)3)

The Fe(CO)3 unit serves as a protecting group for the diene, preventing the diene from participating in Diels-Alder reactions and hydrogenation. The diene is usually deprotected with ceric ammonium nitrate.

These complexes are mainly of academic interest. Some iron tricarbon complexes of cyclopentadienones catalyze hydrogenation, see Knolker complex.

Characterization
IR spectra of these complexes show νCO bands near 2040 and 1969 cm-1. At low temperatures, the lower energy band splits, which has been interpreted as evidence for fluxionality on the IR timescale.