Ring currents in (C6H6)C-r(CO)(3), 1, (cis-1,3-butadiene)Cr(CO)(4) (4), (C6
H6)(2)Cr (5), (C4H4)Fe(CO)(3) (6), and (cis-1,3-butadiene)Fe(CO)(3) (8) hav
e been assessed by sigma-pi disected nucleus independent chemical shift (NI
CS) calculations. Shielding contributions from the C-C(pi) orbitals to the
NICS values reveal that there is no quenching of ring current in the benzen
e 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 magn
etic 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 additi
on, 6 has a negative magnetic susceptibility exaltation (MSE) (diatropic ri
ng current). quite opposite from the large paratropic current calculated fo
r cyclobutadiene (D-2h) itself. Evaluated using "strain corrected" isodesmi
c reactions, 6 has a large ASE in contrast to a large destabilization calcu
lated 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.