Electronic Structure and Electron Transport Properties of Metal Ions 7 



In the biologically important haem compounds the environment seems 

 more nearly octahedral and no "intermediate-spin" derivatives are known. 

 (Magnetic resonance shows conclusively that ferrihaemoglobin hydroxide 

 exists in a mixture of high- and low-spin configurations.) 



Paramagnetic resonance absorption has not been detected in ferrous com- 

 pounds and it is probable that even in the paramagnetic compounds it is 

 outside the range of normal experiments. Paramagnetic resonance experi- 

 ments, however, are perhaps the most sensitive tool at present available for 

 the study of the detailed energy-level scheme both in high- and low-spin 

 Fe+++ complexes. Once this is known inferences can be made about the 

 detailed geometrical structure, and correlations established with other 

 properties. 



PARAMAGNETIC RESONANCE EXPERIMENTS 

 Here I can deal only with the energy-level diagrams deduced (mainly by 

 Griffith, 1956, 1957, 1958) by an analysis of the experimental data of Ingram 

 and co-workers (Gibson and Ingram, 1957; Gibson et al, 1958). The 



Table 2. Resonance characteristics of some haemoglobin derivatives 



theory employed is quite difficult and is given in the original papers. The 

 compounds studied are listed in Table 2. The experiments show unambigu- 

 ously that ferrihaemoglobin at low pH's and ferrihaemoglobin fluoride are 

 high-spin complexes, as is well-known. Ferrihaemoglobin azide and hydroxide 

 are predominantly low spin complexes, although the latter is in thermal 

 equilibrium with the high-spin form. 



So far there has been little detailed discussion of the electronic structure 

 of the high-spin ferric complexes, although valuable information about the 

 orientation of the haem planes in myoglobin and haemoglobin has been 

 obtained. Unpublished calculations (Griffith and Orgel, 1957) suggest that 



