THE ROLE OF THE METAL IN PORPHYRIN 

 COMPLEXES 



By F. P. DwYER 



John Curt in School of Medical Research, Australian National University 



INTRODUCTION 



The more obvious implications of the co-ordination of organic molecules to 

 metal ions : the effective charge reduction of the metal ion, and the polariza- 

 tion of the organic moiety, have tended to become obscured by the wide 

 interest in ligand-metal bond theories. In as much as these of their nature 

 incline to emphasize the separate entities of metal and ligand, attention has 

 been directed away from the properties of the complex as a whole. Sugges- 

 tions that many reactions can occur at the periphery of the metal-porphyrin 

 molecule rather than exclusively at vacant or labile sites on the metal (Williams, 

 1956a; Chance, 1951; King and Winfield, 1959), deserve more serious 

 consideration. The purpose of this paper is to direct attention to properties 

 which are those of the whole complex unit rather than the ligand and metal 

 components. 



The metal-porphyrins are derived from a planar di-acid molecule which 

 differs from the usual planar quadridentates such as 1 : 2-bis(a-pyridyl- 

 methyleneaminoethane) (Fig. 1), by implication of the metal in a closed-ring 

 system which probably contributes considerably to the stability 

 of the complex structure, since the organic molecule cannot 

 be detached point by point and hence unwrapped from the 

 metal. Recent exchange work with the sexadentate molecule 

 1 : 2-propanediaminetctraacetic acid has shown that the six 

 points of attachment to a metal can be broken progressively ^^ ^ 

 in this way (Dwyer and Sargeson, 1960). 



Co-ordination proceeds with the extrusion of two protons and the metal 

 complexes have zero or a small overall positive charge. As a result, since it is 

 obviously easier to detach electrons from complexes with zero or a small 

 positive charge, oxidation is facilitated. This is shown by the redox potential 

 shift on passing from the simple hydrated ions to the iron and manganese 

 complexes. Silver(I) acetate and protoporphyrin react, with the extrusion of 

 a single proton, to yield a silver(I) complex, which spontaneously oxidizes 

 with the extrusion of a second proton. The rather rare formal Ag(II) complex 

 is favoured by the low charge and the planar arrangement of the bonds 

 (Falk and Nyholm, 1958). 



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