Does Cr(CO)3 Complexation Reduce the Aromaticity of Benzene?

Ring currents in (C 6 H 6 )Cr(CO) 3, 1, ( cis -1,3-butadiene)Cr(CO) 4 ( 4 ), (C 6 H 6 ) 2 Cr ( 5 ), (C 4 H 4 )Fe(CO) 3 ( 6 ), and ( cis -1,3-butadiene)Fe(CO) 3 ( 8 ) have been assessed by σ−π disected nucleus independent chemical shift (NICS) calculations. Shielding contributions from the C−C(π) 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 2 h ) 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) π −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 ∼ 1 ∼ 5 > 6 > 4 > 8 .