62 



J. E. Falk and D. D. Perrin 



occupied ^-orbitals on the metal to form molecular orbitals with vacant 

 7T-orbitals on the ligand, so that these electrons gain in stability. (If the 

 77-orbitals are already filled the interaction is repulsive.) Similar interactions 

 can take place between vacant orbitals on the metal and filled 7r-orbitals on 

 the ligand. Effects such as back double bonding cannot be separated from 



True octahedral complex 



ZA 



----B- 



I-9J 



0=0 



/^/ 

 N — i-N 

 Globin 



10 



Square 

 planar 

 complex 



HjO 



/ Pe / 

 N— |-N 



Globin 



05 



Ratio of ligand field in z direction to ligand field in x or y 

 direction 



Fig. 2. Ligand field splitting of ^-orbitals in Metalloporphyrins (qualitative 



only). 



purely electrostatic contributions and they are probably important factors in 

 unsaturated ligands, including porphyrins, oxygen, carbon monoxide, 

 cyanide ion and unsaturated heterocyclic bases, all of which exert strong 

 ligand fields. 



The porphyrin nucleus confers a planar configuration on its metal com- 

 plexes, and any additional co-ordination sites on the metal are perpendicularly 

 above and below this plane (i.e. along the z-axis). This leads with most 

 transition metals to a more or less vertically distorted octahedral structure 

 and, in the metalloporphyrins, the effect of ligands occupying these positions 

 is to split the ^^-orbital energy levels as shown qualitatively in Fig. 2. If the 

 energy separations are greater than the energy needed to pair electrons in the 

 lower energy levels, diamagnetic or low-spin complexes will be formed; 



