The Competition for Electrons:  Aromatic Stabilization in a Six-Membered Ring vs Cyclobutadiene−Iron Complex. The Molecular Structure of Tris(tricarbonylironcyclobutadieno)benzene

The title compound was prepared from hexabromotricyclobutabenzene and diiron nonacarbonyl, and its X-ray structure was measured. The C−C bonds in the six-membered ring are longer than in an typical aromatic moiety (1.443−1.470 Å), suggesting that the system is best regarded as three C(sp 2 )−C(sp 2 ) single bonds connecting three (CO) 3 Fe−cyclobutadiene complexes. It is shown that the effect of complexing tricarbonyliron to a benzocyclobutadiene moiety is not additive. Ab initio calculations and NRT analyses were used to understand the geometrical, electronic, and energetic properties of the systems. It was found that complexation of (CO) 3 Fe to benzocyclobutadiene aromatizes the systems, whereas the opposite is happening upon complexation of three iron moieties to tris(cyclobutadieno)benzene. The study of the energy required for the deformation of the ligands to their geometry in the complex suggests that the iron−cyclobutadiene bond is stronger in the title complex than in (CO) 3 Fe−cyclobutadiene. The title complex shows a small but significant Mills−Nixon effect.