Iron-oxophlorin is an intermediate in heme degradation, and the nature of the axial ligand can alter the spin, electron distribution, and reactivity of the metal and the oxophlorin ring. The structure and reactivity of iron-oxophlorin in the presence of imidazole, pyridine, and t-butyl isocyanide as axial ligands was investigated using the B3LYP and OPBE methods with the 6-31+G* and 6-311+G** basis sets. OPBE/6-311+G** has shown that the doublet state of [(Py)(2)Fe-III(PO)] (where pyridines are in perpendicular planes and PO is the oxophlorin trianion) is 3.45 and 5.27 kcal/mol more stable than the quartet and sextet states, respectively. The ground-state electronic configuration of the aforementioned complex is pi(2)(xz)pi(2)(yz)a(2u)(2)d(xy)(1) at low temperatures and changes to pi(2)(xz)pi(2)(yz)d(xy)(2)a(2u)(1) at high temperatures. This latter electronic configuration is consistently seen for the [(t-BuNC)(2)Fe-II(PO center dot)] complex (where PO center dot is the oxophlorin dianion radical). The complex [(Im)(2)Fe-III(PO)] adopted the d(xy)(2) (pi(xz)pi(yz))(3) ground state and has low-lying quartet excited state which is readily populated when the temperature is increased.