Chapter 15, Problem 43DSP

Cyclobutadiene and (Cyclobutadiene)tricarbonyliron

As we saw in Section $12.17$, cyclobutadiene is antiaromatic and exceedingly difficult to prepare and study. Its successful preparation by Rowland Pettit (University of Texas) in $1965$ demonstrated how transition-metal organometallic chemistry can provide access to novel reactions and structures. His approach was to prepare cyclobutadiene as a transition-metal complex, then destabilize the complex to trigger its dissociation. The sequence for cyclobutadiene begins with the reaction of $\text{cis-3,4- dichlorocyclobutene}$ with diironnonacarbonyl $\left[{\text{Fe}}_2{\left(\text{CO}\right)}_9\right]$. The resulting iron–cyclobutadiene complex satisfies the $\text{18-electron}$ rule, is stable, and undergoes a variety of reactions. Most importantly, oxidation with ceric ammonium nitrate (a source of ${\text{Ce}}^{4+}$) lowers the electron count from $18$ to $16$, causing the complex to dissociate and liberate free cyclobutadiene.

The product of this reaction is formed by an intramolecular Diels–Alder cycloaddition.

What is its structure?