The dynamics of porphyrin ring inversion of a number of Fe(Ill) complexes of octamethyltetraphenylporphyrin, (OMTPP)Fe-III; octaethyltetraphenylporphyrin, (OETPP)Fe-III; octaethyltetra(perfluorophenyl)porphyrin, (F20OETPP)-FeIII; and tetra-beta,beta-tetramethylenetetraphenyl-porphyrin, (TC6TPP)F-III, having either one (Cl-, ClO4) or two [4-(dimethylamino)pyridine, 4-Me2NPy; 1-methylimidazole, 1-Melm; tert-butylisocyanide, t-BuNC; or cyanide, CN-] axial ligands have been characterized by 1D dynamic NMR (DNMR) and 2D H-1 NOESY/EXSY spectroscopies as a function of temperature. The activation parameters, Delta H-inverted iota Delta S-inverted iota, and Delta G(298)(inverted iota), and the extrapolated rate constants at 298 K for three chloride, one perchlorate, and three bis-(4-Me2NPy) complexes as well as [FeOETPP(1 -MelM)(2)]-Cl, (FeOETPP(t-BuNC)(2)]ClO4, and Na[FeOETPP(CN)(2)] have been determined. The results indicate that there is a wide range of flexibility for the porphyrin core (kex(298) = 10-10(7) s(-1)) that decreases in the order TC6TPP > OMTPP > F20OETPP >= OETPP, which correlates with increasing porphyrin nonplanarity. To determine the effect of axial ligands, we calculated the free energy of activation, Delta G(298)(inverted iota) for OETPPFeIII bis-ligated with 4-Me2NPy, 1-Melm, or 4-CNPy (similar to 59 kJ mol(-1)), and for complexes with small cylindrical ligands (t-BuNC and CN-) (similar to 37 kJ mol(-1)). These data suggest that the Delta G(298)(inverted iota) for planar ligand rotation is roughly 20-25 kJ mol(-1).