The reactivity of iron(III) tetraphenylporphyrin pi-cation radical complexes [(TPP.)(FeCl)-Cl-III][SbCl6] (1-1) and [(TPP.)Fe-III(ClO4)(2)] (1-2) with pyridine has been examined by H-1 and H-2 NMR spectroscopy in dichloromethane solution at low temperatures. Evidence for the reaction of the porphyrin ring with pyridine with formation of stable beta-pyridiniumyltetraphenylporphyrin (2) and unstable 5-pyridiniumylisotetraphenylporphyrin (3) and 5,10-dipyridiniumyltetraphenylporphodimethene (4) complexes of iron is presented. Forms 3 and 4 may be directly observed over the temperature range -80 to -40 degrees C. They are new forms of highly oxidized, highly positively charged species that are related to the iron porphyrins. The characteristic patterns of paramagnetically shifted pyrrole resonances (i.e., seven for the beta-substituted form, four for isoporphyrin, and two for porphodimethene iron complexes) are correlated with the symmetry of species and have been used as a definitive indication of porphyrin structural modification. The simultaneous formation of pyridine-substituted iron porphyrins in different oxidation/electronic/spin states has been found. A mechanism of their formation, which involves meso- or beta-substituted sigma-intermediates, has been proposed. The H-1 NMR spectra of the stable, high-spin [(beta-py-TPP)(FeCl2)-Cl-III and low-spin [(beta-py-TPP)Fe-III(py)(2)]Cl-2 products are presented and analyzed. A strong dependency of the H-1 NMR spectra on the counteranion for the low-spin form ([(beta-py-TPP)Fe-III](2+)) has been found and is explained by the formation of a tightly bonded, ionic pair because the complex bears two positive charges.