Is the Aromaticity of the Benzene Ring in the (eta6-C6H6)Cr(CO)3 Complex Larger than that of the Isolated Benzene Molecule?

The aromaticity of the benzene ring in the (eta6-C6H6)Cr(CO)3 complex is analyzed using several indicators of aromaticity based on different physical manifestations of this property. All indices used except NICS show that there is a clear reduction of the aromaticity of benzene upon coordination to the Cr(CO)3 complex. The particular behavior of the NICS index has been analyzed in detail and we have concluded that the reduction of the NICS value in the benzene ring of the ( 6-C6H6)Cr(CO)3 complex is not a manifestation of an increased aromaticity but is due to the ring currents generated by the electron pairs that take part in the benzene–Cr(CO)3 bonding.