The aim of these calculations is to understand the electronic properties of the central iron and charge contributions before and after binding to • NO alongside those of the ligands, • NO and Cl. This understanding is essential for investigating the intra-and intermolecular bonding between each compound and • NO from theoretical and experimental points of view. Hence, the NBO analysis was performed; the NPA charges of the main atoms involved in the binding of ). This explains the higher charges on the porphyrin-nitrogens; however, more specific orbital interactions are taking place, as will be described later. This supports the explanations in the previous sections. Furthermore, an increase in the Eg values within the [Fe(II)-NO] + state was observed in hemin, followed by H-Tyra and finally H-Tyros, while H-Styr complexation with • NO resulted in the lowest Eg, relating to its least stability among all tested compounds.