In • NO, the πg* is partially occupied and has lower energy placing it closer to d-orbitals of Fe . Hence, it was reported that Fe(III)-NO at M=4 spin multiplicity can be expressed as [Fe(II)-NO] + , due to the flow of electronic charge from • NO to Fe-PPIX as detailed by . However, this cannot be considered a general conclusion, where the optimized geometries at the different electronic states revealed significant differences in the interatomic distances and angles following binding to • NO (Table ). Moreover, it was reported that the resulting linear [Fe(II)-NO] + configuration maximizes the bonding of this orbital with the dπ orbital of Fe . However, when Fe-N-O adopt a bent geometry, the double degeneracy of the π* MO of free • NO is removed, where one orbital will have π*y character, and the other one becomes involved in σ-bonding with the dz2 orbital of Fe ). Accordingly, the other set of calculations was performed at M=4 ground states. At [Fe(II)-NO] + (S = 0), the highest affinity of • NO-binding was observed in the case of H-Styr, followed by H-Tyra, then similar lower affinity of the other two complexes. However, at Fe(III)-NO (S = 3/2), the binding of H-Styr to • NO led to the least stable complex structure, while the hemin(NO) complex showed the highest stability, followed by H-Tyra(NO), then H-Tyros(NO). Moreover, even with similar conjugated systems in H-Styr and TPP-Fe, the binding affinities to • NO were different, with generally higher binding energies in the latter case. Furthermore, in Fe(III)-porphyrin-NO adducts, it has been reported that the substitutions on the porphyrin ring have a direct correlation with Fe-NO and N*-O* length, strength and stretching vibrations, a behaviour absent in Fe(II)-porphyrin-CO adducts . Further understanding of these results and whether the various bond lengths calculated affect these energies will be provided later via the NBO charge analysis.