Theoretical studies of low-spin six-coordinate iron(III) porphyrins relevant to cytochromes b:: Variable electronic configurations, ligand noninnocence, and macrocycle ruffling

Low-basicity pyridines and other strong pi-acceptors as axial ligands in six-coordinate iron(III) porphyrins (ferrihemes) result in a (d(xz),d(yz))(4)(d(xy))(1) configuration of the iron(III) center as opposed to the usual (d(xz),d(yz))(3)(d(xy))(2) configuration. Density functional calculations have been used to characterize and visualize these variable electron distributions. For the (d(xz),d(yz))(4)(d(xy))(1) configuration, the porphyrin ligand is strongly noninnocent and has significant a(2u)-type cation radical character due to d(xy)-"a(2u)" interactions. These orbital overlaps may be a significant driving force for porphyrin ruffling. Another factor that favors ruffling appears to be the small ionic radius of low-spin iron(III), with the result that all the ferrihemes studied are significantly ruffled, regardless of electronic configuration.