The DMRG-CASSCF wavefunctions were analyzed in terms of localized orbitals. All DMRG-CASSCF natural orbitals were first localized into ligand-based and Fe-based orbitals. We then used BLOCK2 to decompose the wavefunction into configuration state functions (CSFs). The CSFs were further classified into four resonance structures Fe-NO + , Fe-NO 0 , Fe-NO -, and Fe-NO 2-, allowing us to determine the oxidation state of Fe. We also examined the Mulliken spin populations calculated at the DMRG-CASSCF level of theory. As the DMRG-CASSCF interface in OpenMolcas lacks this functionality, the spin populations were calculated with the ORZ program package 81 in combination with the def2-TZVP basis set. The formal electronic configurations of all complexes are shown in Table .