Electronic Structure and Electron Transport Properties of Metal Ions 5 



we get a diamagnetic complex, but otherwise a paramagnetic one. This then 

 is the explanation we give of the existence of two types of complex known as 

 covalent and ionic in Pauling's theory. We may remark that the theory as 

 presented here is much oversimplified, and that a close correspondence exists 

 between the qualitative features of the present and of Pauling's theory. 



In Table 1 I give the corresponding electron arrangements for other con- 

 figurations of d electrons. In the chemistry of the cytochromes, d"" and d^ 



Table 1. (/-Electron arrangements in octahedral complexes 



configurations are of particular importance but d"^ and d^ may perhaps be 

 relevant. 



One result of considerable interest which emerges from the new treatment 

 is that in a regular octahedral complex d^ ions may have or 4 unpaired 

 spins, but not 2 unpaired spins. Similarly, d^ ions can have 1 or 5 unpaired 

 spins but not 3 (Griffith, 1956). Magnetic susceptibilities corresponding to 

 the intermediate spin states, e.g. for Fe+++, must be due to equilibrium 

 mixtures of molecules with 1 and 5 spins. These may arise through chemical 

 equilibrium of the normal sort, or because A is so close to the critical value 

 for crossing from 1 to 5 spins that the same chemical complex can exist in 

 two different electronic states. We shall see that this is the case in ferri- 

 haemoglobin hydroxide. 



Extensive studies of the spectra of metal ions in solution have established 

 that for a considerable range of metal ions, including Fe++ and Fe+++ ions, 

 the value of A increases whenever a ligand is replaced by a ligand to the right 

 of it in the following series : 



I-, Br-, C1-, F-, H.p, Amines, CN- 



This explains for example why [Fe(H20)6]++ is paramagnetic but 

 [Fe(CN)6]*~ diamagnetic. Similar considerations apply in porphyrin com- 

 plexes and explain, for example, why compounds such as haemoglobin and 



