Does Cr(CO)3 complexation reduce the aromaticity of benzene?

Ring currents in (C6H6)C-r(CO)(3), 1, (cis-1,3-butadiene)Cr(CO)(4) (4), (C6H6)(2)Cr (5), (C4H4)Fe(CO)(3) (6), and (cis-1,3-butadiene)Fe(CO)(3) (8) have been assessed by sigma-pi disected nucleus independent chemical shift (NICS) calculations. Shielding contributions from the C-C(pi) orbitals to the NICS values reveal that there is no quenching of ring current in the benzene ring of 1 or in dibenzene chromium 5. The previously reported paratropic ring current for 1 is shown to be a consequence of latent aromaticity in 1,3-butadiene chromium tetracarbonyl 4 (one of the molecules used in the magnetic susceptibility equation). NICS values, a diatropic ring current, and a positive aromatic stabilization energy (ASE) all point to this aromaticity. NICS values for cyclobutadiene iron tricarbonyl, 6, show moderately sized diamagnetic shielding above the plane of the four-membered ring. In addition, 6 has a negative magnetic susceptibility exaltation (MSE) (diatropic ring current). quite opposite from the large paratropic current calculated for cyclobutadiene (D-2h) itself. Evaluated using strain corrected isodesmic reactions, 6 has a large ASE in contrast to a large destabilization calculated for cyclobutadiene with the same equation. The 1,3-butadiene complex 8 also shows features of three-dimensional aromaticity, NICS(1) -8.7, NICS(1)(pi) -2.8, and a moderate ASE stabilization energy (7.9 kcal/mol, eq 7), but this complex has a negligible MSE of -1.0 ppm cgs. We predict the ring current order to be the following: benzene similar to 1 similar to 5 > 6 > 4 > 8.