Substituent effects of the meso-aryl (Ar) groups on the H-1 and C-13 NMR chemical shifts in a series of low-spin highly saddled iron(III) octaethyltetraarylporphyrinates, [Fe(OETArIP)L-2](+), where axial ligands (L) are imidazole (HIm) and tert-butylisocyanide ((BuNC)-Bu-t), have been examined to reveal the nature of the interactions between metal and porphyrin orbitals. As for the bis(HIm) complexes, the crystal and molecular structures have been determined by X-ray crystallography. These complexes have shown a nearly pure saddled structure in the crystal, which is further confirmed by the normal-coordinate structural decomposition method. The substituent effects on the CH2 proton as well as meso and CH2 carbon shifts are fairly small in the bis(HIm) complexes. Since these complexes adopt the (d(xy))(2)(d(xz), d(yz))(3) ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired electron in one of the metal d(pi) orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin 3e(g)-like orbitals. A fairly small substituent effect is understandable because the 3eg-like orbitals have zero coefficients at the meso-carbon atoms. In contrast, a sizable substituent effect is observed when the axial HIm is replaced by (BuNC)-Bu-t. The Hammett plots exhibit a large negative slope, -220 ppm, for the meso-carbon signals as compared with the corresponding value, +5.4 ppm, in the bis(HIm) complexes. Since the bis((BuNC)-Bu-t) complexes adopt the (d(xz), d(yz))(4)(d(xy))(1) ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled d(xy) orbital interacts with the specific porphyrin orbitals that have large coefficients on the meso-carbon atoms. Thus, we have concluded 1:hat the major metal-porphyrin orbital interaction in low-spin saddle-shaped complexes with the (d(xz), d(zy))(4)(d(xy))(1) ground state should take place between the d(xy) and a(2u)-like orbital rather than between the d(xy) and a(1u)-like orbital, though the latter interaction is symmetry-allowed in saddled D-2d complexes. Fairly weak spin delocalization to the meso-carbon atoms in the complexes with electron-withdrawing groups is then ascribed to the decrease in spin population in the d(xy) orbital due to a smaller energy gap between the d(xy) and d(pi) orbitals. In fact, the energy levels of the d(xy) and d(pi) orbitals are completely reversed in the complex carrying a strongly electronwithdrawing substituent, the 3,5-bis(trifluoromethyl)phenyl group, which results in the formation of the low-spin complex with an unprecedented (d(xy))(2)(d(xz), d(yz))(3) ground state despite the coordination of (BuNC)-Bu-t.