Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 77 



Our results show that as the concentration of pyridine in the solution increases, the 

 affinity of haem for carbon monoxide also rapidly increases (Nakahara and Wang, 

 J. Anier. chem. Soc. 80, 6526, 1958). This observation shows that in dilute solutions 

 the mixed complex pyridine-haem-CO has greater thermodynamic stability than 

 both the complex HaO-haem-CO and the complex pyridine-haem-pyridine. Indeed 

 the affinity of haem for carbon monoxide is so high that when the latter is bubbled 

 through a solution of dipyridine haemochrome in pure pyridine, some mixed complex 

 pyridine-haem-CO is formed. On the other hand no detectable amount of dicarbon- 

 monoxyhaem, OC-haem-CO, was found when aqueous haem solutions, whether 

 with or without added pyridine, were equilibrated with carbon monoxide at even 

 1 atm. pressure (Wang, Nakahara and Fleischer, /. Amer. chem. Soc. 80, 1109, 1958). 



O 



III 12 II 



9 I c 



:Fe' 



810 



:Fe: 



-I,^Z *■ Fe 



r 



49 



671 P) <s? I 



O 



I (A) I (B) 



Fig. 1 



This result leads to two puzzling questions: (1) Why does pyridine increase the 

 affinity of haem for carbon monoxide ? (2) Why does each haem combine with only 

 one carbon monoxide molecule? 



It is possible to find an answer for both of these questions if we assume that 

 Pauling's structure (5) in Fig. 1 contributes substantially to the energy of binding of 

 carbon monoxide by haem. To do this let us omit the more familiar cr-bonds in the 

 mixed complex pyridine-haem-CO, and focus our attention on the 7r-bonds depicted 

 in Fig. 1 . In structure {B), the d^^ orbital of the Fe++ is combined with the p^. orbital 

 of the C-atom to form a molecular-orbital for one of the possible 7r-bonds between 

 Fe++ and the C-atom of carbon monoxide. Similarly, the dy, orbital of the Fe+''" can 

 combine with the py orbital of the C-atom to form a molecular-orbital with maximum 

 density in the FZ-plane for the other possible 7r-bond between Fe++ and the C-atom 

 of carbon monoxide. The superposition of these two molecular-orbitals makes the 

 Fe-C bond cylindrically symmetrical with respect to the Z-axis. The overall effect of 

 this type of 7r-bond formation is to cause the </,j and dy^ electrons of Fe++ to drift 

 towards the positive direction of the Z-axis as illustrated in Fig. 1. 



The effect of added pyridine on the affinity of haem for carbon monoxide can be 

 qualitatively understood by examining the shape of these 7r-bonding orbitals. If the 

 central metal ion is octahedrally bonded to six identical ligands, the d,^ orbital should 

 be highly symmetrical as depicted by diagram (C) in Fig. 2. If, hov/ever, the co- 

 ordinating electron-pairs of the two ligands in the positive and negative directions of 

 the Z-axis are of different strengths, then the d.^. and the d^, electrons of the central 

 metal ion will be displaced toward the direction of the weaker ligand, as depicted by 

 diagram (Z)) in Fig. 2. When the water molecule in carbonmonoxyhaem is replaced 

 by a stronger ligand such as the pyridine molecule, the stronger field of the co-ordinating 

 electron-pair of the pyridine will cause the d.^^ and dy, electrons of Fe++ to shift 

 toward the direction of the CO-molecule on the other side of the haem plane. This 

 will facilitate 77-bond formation between Fe++ and the C-atom of the carbon monoxide 

 and hence increase the affinity of haem for carbon monoxide. Conversely, since the 



