Structure and Bonding
The carbon-carbon bond distances are 1.40 Å within the five membered rings, and the Fe-C bond distances are 2.04 Å. Although X-ray crystallography (in the monoclinic space group) points to the Cp rings being in a staggered conformation, it has been shown through gas phase electron diffraction and computational studies that in the gas phase the Cp rings are eclipsed. The staggered conformation is believed to be most stable in the condensed phase due to crystal packing.
The Cp rings rotate with a low barrier about the Cp(centroid)-Fe-Cp(centroid) axis, as observed by measurements on substituted derivatives of ferrocene using 1H and 13C nuclear magnetic resonance spectroscopy. For example methylferrocene (CH3C5H4FeC5H5) exhibits a singlet for the C5H5 ring.
In terms of bonding, the iron center in ferrocene is usually assigned to the +2 oxidation state, consistent with measurements using Mössbauer spectroscopy. Each cyclopentadienyl (Cp) ring is then allocated a single negative charge, bringing the number of π-electrons on each ring to six, and thus making them aromatic. These twelve electrons (six from each ring) are then shared with the metal via covalent bonding. When combined with the six d-electrons on Fe2+, the complex attains an 18-electron configuration.
Read more about this topic: Ferrocene
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