Aromaticity and ring currents in ferrocene and two isomeric sandwich complexes

Ring currents induced in the ferrocene molecule and its two hypothetical isomers (η4-C4H4)Fe(η6-C6H6) and (η3-C3H3)Fe(η7-C7H7) by an external magnetic field directed along the principal axis are plotted within the ipsocentric approach (at the B3LYP/6-31G∗∗//B3LYP/6-31G∗∗ level). The carbocyclic ligands in all three species are found to be aromatic, i.e. to support individual diatropic ring currents, with formal charges that are consistent with the 4n + 2 rule and the +2 oxidation state of iron.

If ligand aromaticity as determined by ring current is the guiding principle, the two ring-isomers of ferrocene are favourable candidates for stable molecules.Figure optionsDownload as PowerPoint slideHighlights
► Ferrocene has hypothetical isomers where 5/5 rings are replaced by 4/6 or 3/7 rings.
► Ipsocentric calculations show diatropic ring currents in all rings in all 3 isomers.
► All ligands are therefore aromatic.
► Orbital current-density contributions rationalise formal 4n + 2 counts for all rings.