22 F. P. DwYER 



[Fe(CN)6]*" the excess negative charge conferred by six negatively charged 

 donors is supposed to be nearly off-set by three tt bonds from iron to carbon 

 (Pauling, 1938). The charge interaction picture of donor atom and metal is 

 thus quite complex. 



The molecules 4:7-dihydroxy-l :10-phenanthroline (Fig. 4a) and 4:4- 

 dicarboxy-2 : 2'-bipyridine (Fig. 4b) exist normally in the zwitterion forms. 

 In neutral solution very little reaction occurs with iron(II) salts, but in 

 alkaline solution very strong co-ordination occurs not only because the 

 nitrogen atoms are now free, but because at least two protons have been 



Fig. 4 



detached and neutralized. The tris-chelatQ iron complexes have therefore 

 zero charge or are anions depending on the pH. At the biological pH the 

 carboxylic acid side-chains in many porphyrins, e.g. protoporphyrin, haemato- 

 porphyrin, etc., make some contribution to the stability of the complex. 

 Quite independently of other factors, such groups when sufficiently acidic 

 promote oxidation by reducing the overall positive charge. 



OXIDATION STATE OF THE METAL IN COMPLEXES 



In the absence of obvious oxidizing or reducing conditions, the co-ordina- 

 tion of a ligand to a metal ion is taken to involve no change in the oxidation 

 state. The number of unpaired electrons, but not necessarily their location, 

 can be obtained from magnetic moment measurements. The validity of 

 Hund's Rule, which usually needs to be invoked to translate magnetic data 

 into the oxidation state, has been frequently questioned when the magnetic 

 evidence is at variance with the chemical properties. 5/5'(dimethylglyoxime)- 

 copper has the moment characteristic of one unpaired electron, but from the 

 absence of metal-metal interaction in the crystalline state, it has been deduced 

 that the unpaired electron is mostly located on the ligands (Rundle, 1954). 

 It would not be unreasonable to think of the Cu atom as in the -f3 diamag- 

 netic state and the ligands as reduced. The observation that the 1 -electron 

 oxidation of copper phthalocyanine removes an electron from the ligand and 

 not the metal suggests a similar disposition of the unpaired electron (Cahill 

 and Taube, 1951). 



The elaborate system of conjugated double-bonds in iron protoporphrin 

 makes it feasible that oxidation could yield stable seini-quinone structures 

 without affecting the oxidation state of the iron. Recently, Gibson and 

 Ingram (1956), using the electron spin resonance method, showed that the 



