Some Physical Properties and Chemical Reactions of Iron Complexes 49 



(1) The absorption spectrum of Fe(DMG)2 (imldazole)2 does not change 

 with pH from 6-11 except in intensity (Croft and WilUams). 



(2) The complex Fe++(DMG)2 (imidazole)2 is extractable into /j'o-amyl 

 alcohol (Croft and WilUams). 



(3) The complexes Cu++(histidine) and Cu++(histidine)2 show no ioniza- 

 tion of the type (2) (Leberman and Rabin, 1959; James and Wilhams, 

 unpublished). 



(4) Amongst biological molecules, Fe++-cytochrome c has no ionization 

 in the expected range (Lemberg and Legge, 1949). 



In the presence of oxygen it is observed that the lower ^K is raised. We 

 accept Pauling and Coryell's (1936) explanation, that this implies that the 

 oxygen inserts itself between the imidazole and the Fe++ ion. It would 

 appear that the second p^is not due to the imidazole groups at all. The ^K 

 shift could be due to the ionization of an — NH3+ group in the protein, the 

 basicity of which was altered by the change in protein stereochemistry on 

 deoxygenation of haemoglobin. 



Again, we have no evidence to show that Fe+++(NiOX)2 (imidazole)2 

 undergoes any ionization up to pH 10-0. The titration of the complex with 

 alkali, its insolubility in organic solvents, and its absorption spectrum all 

 indicate that the ligand is imidazole and not the imidazole anion. 



CHEMICAL REACTIONS OF IRON COMPLEXES 



It has always been our intention to proceed from a detailed study of the 

 physical properties of ferrous and ferric complexes to a study of their chemical 

 reactions. We have now made a start with the latter phase of this work. 



Reactions of Molecular Oxygen 



Oxygen can either combine with ferrous complexes (oxygenation) or 

 oxidize them (autoxidation). In biological systems both reactions occur. We 

 have observed both reactions also in the chemistry of the complexes 

 Fe++(DMG)2X2 and Fe++(NiOX)2X2, where DMG is dimethylglyoxime. 

 Our studies show that in the model systems the oxygenated complexes 

 Fe(DMG)2X02 are not stable if X is readily exchanged for water or if X is a 

 group containing labile hydrogen. The reactions can be illustrated by 

 examples. When X is imidazole or pyridine the oxygenated complex is 

 stable with certain qualifications. The replacement of the ligands X in the 

 imidazole and pyridine complexes is much slower than in other complexes. 

 When X is water, hydrazine, ammonia, aniline or other substituted amines, 

 or sterically hindered pyridines or imidazoles (e.g. histidine) the oxygenated 

 complex is not as stable but undergoes autoxidation. The replacement of 

 ligands in these complexes is more rapid. Autoxidation is different in different 

 cases giving oxidation of the group X in some cases (e.g. N2H4) and not in 



