Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 67 



as would be required for any general correlation of spectra with redox 

 potential. 



OXIDATION-REDUCTION POTENTIALS 

 Factors affecting the oxidation-reduction potentials of metal complexes 

 (Perrin, 1959b) include: 



(i) Purely electrostatic effects of attractions between ions or dipoles of 

 opposite charge. Thus if the ligand is an anion this will always favour 

 the higher valent state of the metal and the potential will be less than 

 for the corresponding free metal ions in water. This is because the 

 standard oxidation-reduction potential, E^, of any pair of ligand- 

 metal complexes is related directly to the stabiUty constants of the 

 complexes by the identity, 



2'3036RT^, ,, , .. ^ 



E1 = Em- 7 7^ (log ^M"^ - log Kj^m+) 



(n — m)r 



where E^ is the potential of the free metal ions. 



(ii) Back-double-bonding. Because this involves the removal of electrons 

 from the vicinity of the positively charged metal ion, the effect is 

 always greater for the lower valent state of the metal, so that it tends 

 to raise the oxidation-reduction potential. 



(iii) Ligand field stabilization energies. The difference in L.F.S.E. varies 

 with the particular pairs of cations and the ligands, as discussed in 

 Dr. Orgel's paper (loc. cit.). For example, in weak ligand fields 

 ferrous ion, but not ferric ion, is stabilized in this way : this raises the 

 potential. On the other hand, in manganous, manganic systems 

 manganic ion is stabilized but not manganous ion, so the potential 

 is lowered. 



(iv) The acid dissociation constants (p/sTa's) of the ligands. In many 

 series of closely related ligands the stabiUty constants of metal com- 

 plexes vary with p^^ ^^ ^^ approximately linear fashion : log K 

 ^^ apAr„ •\- c. Such a relation would be expected if the factors 

 governing the binding of protons and cations by ligands were similar. 

 From a simple electrostatic model it has been predicted that a should 

 increase with increasing cationic charge (Jones et al., 1958). As a 

 direct consequence, the oxidation-reduction potentials of metal 

 complexes in which the ligands are sufiiciently similar should decrease 

 linearly with increasing ipK^. For several series of iron complexes 

 this has been found to be the case (Perrin, 1959b). 



In the porphyrins, electron-withdrawing substituents at the peripheral 

 carbons increase the acid strength of the porphyrin (lower the pATJ and hence 

 raise the oxidation-reduction potential of the metal complex. Martell and 



