Electronic structure of selected FeNO7 complexes in heme and non-heme architectures: a density functional and multireference ab initio study.

The multiconfigurational CASSCF/CASPT2 approach, along with various functionals of density functional theory, is applied to selected iron(II)-nitrosyl ({FeNO}(7)) complexes, both with heme and nonheme groups. The energetics of the lowest doublet and quartet spin states at the correlated ab initio (CASPT2) level is presented for the first time. Comparison of the CASSCF and (unrestricted) DFT spin densities indicates that the nonhybrid functionals yield the spin densities most closely to the ab initio ones. The analysis of the multiconfigurational CASSCF wave function in terms of the localized active orbitals allows one to resolve the nature of Fe-NO bonding as a mixture of Fe(II)-NO(0) and Fe(III)-NO(-) resonance structures (in comparable contributions) for both spin states and various ligands.