L, E. Orgel 



ferrihaemoglobin are low-field but carboxy-haemoglobin and cytochrome c 

 high-field complexes. However, to understand the finer details of haem- 

 protein interactions we must develop the theory further, 



LIGAND-FIELD THEORY OF METAL PORPHYRIN COMPLEXES 

 The immediate environment of a metal ion in a porphyrin or phthalo- 

 cyanine derivative consists of the four coplanar nitrogen atoms of the 

 macrocyclic ring and up to two further groups. In nickel phthalocyanine, for 

 example, the fifth and sixth co-ordination positions are empty, in ferric 

 phthalocyanine chloride the metal atom is probably five co-ordinated, while 

 in the biologically important haem derivatives the Fe++ or Fe+++ ions are 

 believed to be six co-ordinated. 



We consider therefore the way in which the energies of the different orbitals 

 are changed when, in a regular octahedral complex, the two ligands on the 



Distortion — >- Distortion >- 



(a) (b) E^., ' 



Fig. 5. Energy level diagram showing the effect of tetragonal distortions; 

 (a) electrostatic theory; (b) a possible consequence of double bonding. 



z axis are gradually removed. (This is formally equivalent to replacing two 

 ligands by groups which produce smaller crystal fields.) In Fig. 5a we illus- 

 trate the results of calculations based on the electrostatic theory, and in Fig. 

 5b the way in which these calculations might be modified by covalent bonding 

 effects. The principal features, namely the splitting far apart of the d^i_y2 

 and d^i orbitals and the maintaining of the degeneracy of the d^.^ and dy^ 

 orbitals are unaffected by covalent bonding, but the order of the d^^y orbital 

 and the d^.^ and dy^ orbitals might be altered. 



If the d^i. orbital becomes much more stable than the dj,2_yi orbital all the 

 electrons may crowd together in the bottom /ow orbitals. Then we would 

 get 0, 1, 2 and 3 unpaired electrons in d^, d'', d^ and d^ ions, respectively. 

 These extreme conditions seem to apply in nickel phthalocyanine (diamag- 

 netic), cobalt phthalocyanine (one unpaired electron) and ferric phthalo- 

 cyanine chloride (three unpaired electrons). 



