A simple model for the electronic structure of Iiganded hemoglobins

Why does dioxygen bind to iron and cobalt porphyrins in an end-on bentbond fashion as in (4.37) and (4.46)? Why does carbon monoxide bind in a linear manner (Equation 4.40)? Why are six-coordinate dioxygen and carbonmonoxide adducts more stable than five-coordinate ones? A unified picture of ligand binding that addresses these questions is important in understanding properly the specific case of dioxygen binding to hemoglobin and related systems. The splitting of the metal d orbitals for a four-coordinate metalloporphyrin is shown in the center of Figure 4.22. These orbitals contain some porphyrin character and are antibonding with respect to metal-porphyrin bonds. As shownin Figure 4.16, the primary effect of a single a-donor axial ligand, such as pyridine or 1-methylimidazole, is to elevate the energy of the antibonding dz2 and lower the energy of the dx 2 _ y2 orbital and hence lead to a high-spin species in place of the intermediate-spin four-coordinate one. Thus, for simplicity in highlighting interaction of the metal center with the diatomic o--donor: 7T-acid ligands CO, NO, and O2 , the perturbations wrought by primarily o--donor ligands, such as 1-methylimidazole, are omitted. For the corresponding cobalt(II) compound, there is an additional electron. The diatomic ligands of interest share a qualitatively similar molecular orbital scheme. The filling of orbitals for CO is shown on the left-hand side. Dioxygen, which is shown on the right-hand side, has two more electrons than CO; these occupy the doubly degenerate 7T* orbitals. Quantitative calculations show that the energy of the 7T* orbitals decreases monotonically from CO to NO to O2 , indicating increasing ease of reduction of the coordinated molecule, a feature that has not been included in the diagram. Only those interactions of molecular orbitals that have appropriate symmetry and energy to interact significantly with the metal d orbitals are shown.
 

Two extremes are shown in Figure 4.22 for the interaction of a diatomic molecule A-B with the metal center: a linear geometry on the left and a bent geometry on the right. A side-on geometry is omitted for the binding of O2 to a COIl or Fell porphyrin, since this would lead to either an MIll side-on superoxo or an M1V peroxo species; both these modes of coordination to these metals are currently without precedent.
 

Linear diatomic metal bonding maximizes the metal-d7T to ligand-p7T* bonding. When a ligand coordinates in a bent manner, axial symmetry is destroyed, and the degeneracy of the ligand P7T* orbitals is lifted. One P7T* orbital is now oriented to combine with the metal dz2 orbital to form a 0- bond, and the other is oriented to combine with dxz and dyz orbitals to form a 7T bond. A bent geometry for the diatomic molecule will result when either or both of the metal dz2 or the ligand P7T* orbitals are occupied, since this geometry stabilizes the occupied dz2 orbital in the five-coordinate complex. Thus O2 binds in a strongly
bent manner to COIl and Fell porphyrins; NO binds in a strongly bent manner to COIl porphyrins; CO binds in a linear fashion to Fell porphyrins.
 

The interaction of NO with Fell porphyrins and CO with COIl porphyrinsthe resultant species are formally isoelectronic-is more complicated. The degree of bending seen in Fell(TPP)(NO) is midway between the two extremes. III For CO the higher-energy P7T* orbitals lead to a greater mismatch in energy between the dz2 and P7T* orbitals, and less effective 0- bonding. In EPR experiments the odd electron is found to be localized in a molecular orbital with about 0.87 metal dz2 character for the five-coordinate Co-CO adduct, as expected for a nearly linear geometry. 129 On the other hand, for the Fe-NO adduct the metal dz2 character of the odd electron is about 0.4 to 0.5; 155 a somewhat bent geometry (140°) is observed in the crystal structure of Fe(TPP)(NO). Because the CO ligand is a very weak 0- donor, the Co-CO species exists only at low temperatures.
 

Only qualitative deductions can be made from this model about the extent of electron transfer, if any, from the metal onto the diatomic ligand, especially for dioxygen. The higher in energy the metal dz2 orbital is with respect to the dioxygen P7T* orbitals, the closer the superoxo ligand comes to being effectively a coordinated superoxide anion. With an additional electron, the dioxygen ligand

in Co-Oz complexes can acquire greater electron density than it can in Fe-Oz complexes.
 

From the diagram it may be inferred that a ligand with very strong 7T-acceptor properties will lower the energy of the dxz and dyz orbitals through strong (dw dyz)-7T* interaction. The resultant energy gap between these two orbitals and the other three metal d orbitals may be sufficient to overcome the energy involved in spin-pairing, and hence lead to five-coordinate low-spin species, as happens for complexes containing phosphines and carbon monosulfide.9