Introduction of a single meso substituent into ClFe(III)(OEP) or K[(NC)(2)Fe(OEP)] results in significant changes in the geometric and/or spectroscopic properties of these complexes. The mono-meso-substituted iron(III) complexes ClFe(III)(meso-Ph-OEP), ClFe(III)(meso-n-Bu-OEP), ClFe(III)(meso-MeO-OEP), ClFe(III)(meso-Cl-OEP), ClFe(III)(meso-NC-OEP), ClFe(III)(meso-HC(O)-OEP), and ClFe(III)(meso-O(2)N-OEP) have been isolated and characterized by their UV/vis and paramagnetically shifted (1)H NMR spectra. The structures of both ClFe(III)(meso-Ph-OEP) and ClFe(III)(meso-NC-OEP) have been determined by X-ray crystallography. Both molecules have five-coordinate structures typical for high-spin (S = 5/2) iron(III) complexes. However, the porphyrins themselves no longer have the domed shape seen in ClFe(III)(OEP), and the N(4) coordination environment possesses a slight rectangular distortion. These high-spin, mono-meso-substituted iron(III) complexes display (1)H NMR spectra in chloroform-d solution which indicate that the conformational changes seen in the solid-state structures are altered by normal molecular motion to produce spectra consistent with C(s) molecular symmetry. In pyridine solution the high-spin six-coordinate complexes [(py)ClFe(III)(meso-R-OEP)] form. In methanol solution in the presence of excess potassium cyanide, the low-spin six-coordinate complexes K[(NC)(2)Fe(III)(meso-R-OEP)] form. The (1)H NMR spectra of these show that electron-donating substituents produce an upfield relocation of the meso-proton chemical shifts. This relocation is interpreted in terms of increased contribution from the less common (d(xz),d(yz))(4)(d(xy))(1) ground electronic state as the meso substituent becomes more electron donating.