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 (η 6 -C 6 H 6 )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 (η 6 -C 6 H 6 )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.