Density functional studies of iron-porphyrin cation with small ligands X (X: O, CO, NO, O2, N2, H2O, N2O, CO2).

Molecular structure of the iron porphyrin cation [FeP]+ with small ligands X (X: O, CO, NO, O2, N2, H2O, N2O, CO2) are studied employing density functional theory (DFT) methods with the exchange-correlation (XC) functionals OPBE and B3LYP using the LANL2DZ basis set. The relative spin state energies and bond dissociation energies of all of the complexes are presented at their optimized geometries. The low-spin (S = 1/2, S = 0) state is found to be the lowest energy states for the [FePO]+, [FePCO]+, and [FePNO]+ complexes whereas the high-spin (S = 5/2) state has the lowest energy for the [FePO2]+ complex. The intermediate-spin (S = 3/2) state is found to be the lowest energy states for the [FePN2]+, [FePH2O]+, [FePN2O]+, and [FePCO2]+ complexes which exhibit the same relative spin-state energy ordering: (S = 3/2) < (S = 5/2) < (S = 1/2) as isolated [FeP]+, and the Fe-ligand bonding is very weak. The calculated bond dissociation energy using the OPBE XC-functional method has shown the following order for the lowest energy spin state: N2O < CO2 < N2 < O2 < H2O < CO < NO < O. This level of theory was previously shown to be the only DFT method capable of correctly predicting the spin ground state of iron compounds, and we find similar good performance of OPBE XC-functional in the current study.