Some Physical Properties and Chemical Reactions of Iron Complexes 53 



Williams, R. J. P. (1955). Special Lectures in Biochemistry, University College, London. 



H. K. Lewis & Co., London. 

 Williams, R. J. P. (1956). Chem. Rev. 56, 299. 

 Williams, R. J. P. (1958). Disc. Faraday Soc. 26, 123. 

 Williams, R. J. P. (1959). TIw Enzymes (Ed. by P. D. Boyer, H. Lardy & K. Myrbiick), 



vol. I, p. 391, Academic Press, New York. 



DISCUSSION 



Oxidation-reduction Potentials of Haem Coinpounds 



Perrin : I should like to ask Williams what evidence he has for believing that with increasing 

 ligand basicity the redox potentials of ferrous/ferric couples pass through a maximum. 

 The only experimental evidence that I have so far found suggests, on the contrary, 

 that in a related series of ligands there is a roughly linear dependence of redox potential 

 on the pA^ of the ligand: the potential decreases continuously as the ligand pAT 

 increases. This is true, for example, of iron complexes with a number of 5-substituted- 

 o-phenanthrolines (Brandt and GuUstrom, 7. Amer. diem. Soc. 74, 3532, 1952). Other 

 systems where linearity is found include the 1 : 1 iron-amino-acid complexes (Perrin, 

 J. chem. Soc. 290, 1959) and the iron complexes of 8-hydroxyquinolines and polyaza- 

 1-naphthols (Albert and Hampton, /. chem. Soc. 505, 1954; Albert, Biocliem. J. 54, 

 646, 1953). The reported potential of 0-7 V for the iron complex of 4-hydroxy-3- 

 carbethoxy-o-phenanthroline (Hale and Mellon, /. Amer. chem. Soc. 72, 3217, 1950) 

 appears at first sight to be anomalously low but I think it can be readily explained. 

 In heterocyclic compounds a hydroxyl group in a gamma position relative to a 

 nitrogen makes two tautomers possible — an enol form, where the H is on the oxygen, 

 and an amide form where the H is on the nitrogen. Contrary to the way the formulae 

 are generally written, the amide form is greatly favoured relative to the enol form 

 (for example, in 4-hydroxyquinoline the ratio is 24,000 to 1 (Albert and Phillips, 

 J. chem. Soc. 1294, 1956) and it would be expected to be even higher for a 1-hydroxy- 

 o-phenanthroline). I suggest that this effect, together with the sparing solubility of 

 the substance, leads to insufficient complex formation to prevent extensive hydrolysis 

 of ferric ion and this is what causes the potential to be so low. It should be pointed 

 out (1) that this system did not behave reversibly and (2) that the corresponding 

 substance without the carbethoxy group was more soluble and gave a higher and 

 reversible potential that was closer to the expected value (Hale and Mellon, loc. cit.). 



In the metalloporphyrins and related substances also there seems to be, in all cases 

 where both are known, a continuing decrease in redox potential with increasing pA' 

 of the ligand. Some examples are given in Tables 5 and 6 of the paper of Falk and 

 Perrin (this volume, p. 69). 



It seems reasonable to suppose that the change from high-spin to low-spin in a 

 complex does not result in any great alteration in the nature of the metal-ligand 

 bonds. The main differences would lie in the extent to which the metal's Id orbitals 

 are made more or less available to take part in bond formation. As discussed more 

 fully elsewhere (Perrin, Rev. pure andappJ. Chem., 9, 257, 1959) I believe these and ligand 

 field stabilization energy changes for any series of iron complexes would make only 

 a slight contribution to change in their overall stabilities and hence their redox 

 potentials. 



Could Williams give any examples of iron complexes where the redox potentials in 

 any series do, as he suggests, increase with the pA" of the ligand ? 

 Williams: The arguments I use in discussing redox potentials are set out in full in my 

 papers. There is as yet no direct evidence for the maximum Perrin discusses. On the 

 other hand for the series of ligands H2O, pyridine, histidine, ammonia, there is good 

 evidence that ^ohas little relationship to pA: of the base (see Dwyer, this volume, p. 25, 

 and Falk and Perrin, this volume, p. 69). In both cases E^ goes through a maximum 

 with pAT. I do not accept either the discussion of iron phenanthroline or iron 8-hydroxy- 

 quinoline complexes given by Falk and Perrin {loc. cit.) and in the question, but prefer 



