The result shows that the surface of the triplet causes a PHmd—CO minimum and that 3,5MECP is the lowest energy path to PHmd—CO. It is found that this CO binding involves a low barrier of 0.931 kcal/mol because of the need to change from a bound triplet state to the Hmd quintet ground state. <...> Infrared spectroscopic data have revealed two CO molecules as iron ligands [ 5 ]. <...> Mцssbauer spectroscopic data showed that iron was in a low-spin, low-oxidation state [ 7 ]. <...> Harvey et al. [ 9 ] investigated the intrinsic barrier to the CO geminate recombination with heme compounds by the DFT calculation. <...> In this study, we reported the reaction mechanism of CO binding with PHmd by the DFT calculation (Fig. 1). <...> We found that a recombination of PHmd (Fig. 1, B) with CO is a spin-forbidden reaction between single CO and quintet PHmd to produce the PHmd—CO triplet, which results in a spin-state change in the central iron atom from high-spin FeII (S = 2) to low-spin FeII (S = 1). <...> The spin-forbidden reaction barrier to a recombination of PHmd with CO is located by characterizing the spin-state surface crossing behavior, i.e., by finding the minimum energy crossing point (MECP) between the surfaces [10—12 ] with a recently developed algorithm. <...> We used 6-31G(d,p) for Fe and the Ahlrichs VTZ basis set for all other atoms. <...> For PHmd coordinated with five ligands, the quintet state was calculated to be 3.215 kcal/mol and 6.213 kcal/mol more stable than the singlet and triplet states, respectively, which indicates that the quintet state is the ground state. <...> The lengths of Fe—CO bonds in PHmd—H2 and PHmd—COOH of 1.789 Е and 1.792 Е, respectively, are in good agreement with the experimental data [ 16, 17 ]. <...> However, for PHmd—CO coordinated with three CO molecules with a central Fe atom, the total energy of the triplet state is –2744.28177432 a.u. lower than that of the singlet (–2744.27422082 a.u.) and quintet states (–2774.26033807 a.u.), indicating that the triplet state is the ground state. <...> It is obvious that the coordination of third CO to Fe leads to a change in the spin state from quintet to triplet. <...> This phenomenon was also found in a recent DFT <...>