Some Physical Properties and Chemical Reactions of Iron Complexes 43 



Tbeorell (1942), Williams (1955, 1956) and Scheler, Schoffa and Jung (1957) 

 have drawn attention to the band-shifts as the magnetic moment of the ferric 

 complexes changes. The Soret band moves to longer v^avelengths in the 

 lower moment complexes. Here we point out that in ferrous complexes the 

 band moves in the opposite direction. The lower the moment of a Fe++PX2 

 complex (P is porphyrin and Xg the further co-ordinating ligands) the shorter 

 the wavelength of the absorption band. Now if we plot the difference in 

 Soret band position Fe++-Fe+++ (AA) against the sum of the magnetic 



^ 

 ^ 



20 



70 



12-0 



Fig. 1 . The relationship between the sum of the magnetic moments of the ferrous 



and ferric haemoglobin complexes in the presence of the additional ligands shown 



and the difference in position of the Soret band in the two complexes. 



moments (/fFe++ -f ^Fe+++) we can obtain a qualitative guide to the character 

 of iron porphyrin complexes from their spectra (Fig. 1). A large value of 

 AA implies ionic complexes. On this basis we have the series of complexes 

 of decreasing ionic character: peroxidase = myoglobin = haemoglobin 

 > cytochrome a^ > cytochrome d > cytochrome a > cytochrome h > cyto- 

 chrome c. This series was devised by entirely different reasoning in earlier 

 papers (Williams, 1958, 1959). The extreme members are ionic and covalent 

 respectively (from magnetic observations), whereas the intermediate members 

 are apparently mixtures of two spin forms in equilibrium (from spectroscopic 

 evidence). 



A possible explanation of the opposed band shifts in the ferrous and ferric 

 series is that the two cations are differently affected in their character on 

 going from the high-spin to the low-spin state. If we take the Soret transition 

 to be either an « -> tt or a tt' -> tt transition in which the electrons are more 

 concentrated on the nitrogen in the ground state than in the excited state, 



H.E. — VOL. I — E 



