The effect of distal histidine on ligation of NO to ferrous and ferric-heme, has been investigated with the high-level density functional theoretical (DFT) method. It has been predicted that the distal histidine significantly stabilizes the interaction of NO ferrous-heme (by −2.70 kcal mol⁻¹). Also, water hydrogen bonding is quite effective in strengthening the Fe–NO bond in ferrous heme. In contrast in ferric heme, due to the large distance between the H₂O and O(NO) and lack of hydrogen bonding, the distal histidine exhibits only a slight effect on the binding of NO to the ferric analogue. Concerning the bond nature of Fe^II–NO and Fe^III–NO in heme, a QTAIM analysis predicts a partially covalent and ionic bond nature in both systems.