Cyclopentadienyliron Dicarbonyl Dimer - Structure

Structure

In solution, Cp₂Fe₂(CO)₄ can be considered a dimeric half sandwich complex. It exists in three isomeric forms: cis, trans, and unbridged. These isomeric forms are distinguished by the position of the ligands. Cis and trans differ in the relative position of C₅H₅ (Cp) ligands. And for both isomers, two CO ligands are terminal whereas the other two CO ligands bridge between the iron atoms. In the unbridged isomer, no ligands bridge between iron atoms — the metals are held together only by the Fe-Fe bond. Cis and trans isomers are the more abundant.

In solution, the three isomers interconvert. The phenomenon of rapidly interconverting structures is called fluxionality. Fluxional process for cyclopentadienyliron dicarbonyl dimer is so fast that only averaged single signal is observed in H NMR spectrum. However, the fluxional process is not fast enough for IR spectrum. Thus, three absorptions are seen for each isomer. The ν_CO bands for bridging CO ligands are around 1780 cm-1 whereas νco bands for terminal CO ligands are about 1980 cm-1.

The solid state of molecular structure of both cis and trans isomers have been analyzed by X-ray and neutron diffraction. The Fe-Fe separation and the Fe-C bond lengths are the same in the Fe₂C₂ rhomboids, an exactly planar Fe₂C₂ four-membered ring in the trans isomer versus a folded rhomboid in cis with an angle of 164°, and significant distortions in the Cp ring of trans isomer reflecting different Cp orbital populations. Although older textbooks show an Fe-Fe bond, theoretical analyses indicated the absence of a direct Fe-Fe bond.