The aromatic/anti-aromatic behavior of the cyclopentadienyl anion (Cp-), bis(η5-cyclopentadienyl)iron(II) (Fe(Cp)2), as well as of the Jahn-Teller (JT) active cyclopentadienyl radical (Cp•) and bis(η5-cyclopentadienyl)-cobalt(II) (Co(Cp)2) were investigated using density functional theory (DFT) calculations of the nuclear independent chemical shifts (NICS). According to the NICS values, pentagon ring in Fe(Cp)2 is more aromatic than that of the isolated Cp-. The NICS parameters were scanned along the Intrinsic Distortion Path (IDP) for Cp• and Co(Cp)2 showing anti-aromaticity, which decreased with increasing deviation from the high symmetry D5h to the low symmetry (LS) C2v. Changes in the NICS values along the IDP revealed that Co(Cp)2 in the LS nuclear arrangement has aromatic character, in contrast to the case of Cp•.
Andjelković, L., Perić, M., Zlatar, M., & Gruden-Pavlović, M. (2015). Nucleus-independent chemical shift profiles along the intrinsic distortion path for Jahn-Teller active molecules. Study on the cyclopentadienyl radical and cobaltocene. Journal of the Serbian Chemical Society, 80(7), 877–888. https://doi.org/10.2298/JSC141107025A