THE ELECTRONIC STRUCTURE AND ELECTRON 



TRANSPORT PROPERTIES OF METAL IONS 



PARTICULARLY IN PORPHYRIN COMPLEXES 



By L. E. Orgel 



Department of Theoretical Chemistry, University Chemical Laboratory, 



Cambridge 



INTRODUCTION 

 The electronic structure of metal-porphyrins has often been discussed in 

 terms of the valence-bond theory as developed by Pauling (1940). In recent 

 years a related but more quantitative theory of metal complexes has been 

 developed and is known as ligand-field theory (Griffith and Orgel, 1957; 

 Moffitt and Ballhausen, 1956). The first part of this paper attempts to give 

 an elementary account of this theory insofar as it is of interest to biochemists 

 working with haem compounds. In particular I shall discuss the relevance 

 of magnetic susceptibility and magnetic resonance data. In the later part of 

 my paper I shall discuss some recent work on electron-transfer processes 

 involving metal ions, and also show how the electronic structures of the 

 haem enzymes may be relevant to the types of electron-transfer which can 

 take place. 



LIGAND-FIELD THEORY OF REGULAR OCTAHEDRAL 

 COMPLEXES 



The five 3<r/ orbitals are fundamental to any discussion of the properties of 

 iron porphyrins and related compounds. They are illustrated in Fig. 1. The 

 fundamental idea of the ligand-field approach is that the effect of the environ- 

 ment of a metal ion is not the same for all the d orbitals and that many 

 of the characteristic properties of transition-metal compounds depend on 

 just this differential effect of the environment. 



We consider first the case of regular octahedral co-ordination. Figure 1 

 shows that the d^2_yz orbital is directed along the x and y axes while the 

 d^y orbital is directed along the bisectors of these axes. If we suppose that 

 the ligands lie along the axes then clearly the d^^._yi orbital comes closer to 

 them than does the d^y orbital. If the ligands are negatively charged or are 

 dipolar with the negative ends of their molecular dipoles pointing towards 

 the metal ion then clearly the d^y orbital is more stable than the dj.i^yi orbital, 

 since an electron in the former is repelled less by the ligands. 



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