The preparation and characterization of two crystalline forms of [Fe(TMP)(5-MeHIm)(2)]C1O(4) with distinctly different molecular structures are reported. Crystal structure analysis shows that paral-[Fe(TMP)(5-MeHIm)(2)]C1O(4) has the axial imidazole ligands arranged in a relative parallel orientation lover a slightly S-4-ruffled porphyrin core) and perp-[Fe(TMP)(5-MeHIm)(2)]C1O(4) has the axial imidazole ligands arranged in a relative perpendicular orientation (over a considerably S-4-ruffled porphyrin core). The two species have different Mossbauer and solid-state EPR spectra. The small quadrupole splitting (Delta E-q = 1.78(1) mm/s, 120 K) and a single observable EPR g(max) value (3.43 at 4.2 K) for perp-[Fe(TMP)(S-MeKIm)(2)]C1O(4) are indicative of the relative perpendicular arrangement of the axial ligands. The larger quadrupole splitting (Delta E-q =1.78(1) mm/ s, 120 K) and rhombic g-tensor (g(1) = 2.69, g(2) = 2.34-2.43, and g(3) = 1.75) in the solid state and in frozen DMF-acetonitrile 3:1 (g(1) = 2.64, g(2) = 2.30, and g(3) = 1.80) at 4.2 K for paral-[Fe(TMP)(S-MeHIm)(2)]C1O(4) are indicative of a relative parallel axial ligand orientation. The actual axial ligand dihedral angles are Delta phi = 76 degrees and Delta phi = 26 or 30 degrees for perp- and paral-[Fe(TMP)(S-MeHIm)(2)]C1O(4), respectively, and thus the dihedral angle at which the EPR spectral type changes from large g(max) to rhombic must be 30 < Delta phi < 76 degrees. Because the porphyrin and axial ligands are similar for both crystalline forms of [Fe(TMP)(S-MeHIm)(2)]ClO4, a more direct correlation between molecular and electronic structure has been established. Molecular mechanics calculations indicate that nonbonded interactions between the axial ligands and meso-mesityl groups of [Fe(TMP)(5-MeHIm)(2)](+) destabilize a relative parallel orientation for the axial ligands, yet the parallel orientation is observed in all frozen solution samples as confirmed by EPR investigations. This is believed to be due to the competing stabilization of the electronic state of the rhombically distorted parallel complex with an energy stabilization of 2.8-3.7 kcal/mol, as compared to the energy destabilization of 2.6 kcal/mol obtained from MM calculations.