Ferrihaemoprotein Hydroxides 



109 



the number of unpaired electrons remains the same, whereas if the lower ?29 

 orbitals are filled preferentially the number of unpaired electrons is necessarily 

 reduced. For example, with ferrous and ferric complexes the former con- 

 figuration gives 4 and 5 unpaired electrons, and the latter, zero and 1 unpaired 

 electrons respectively, as shown in Figs. 3a and 3c. The former configuration 



Fig. 2. An octahedral co-ordination complex, MLg, showing the x-, y-, and 

 z-axes with respect to which the orientation of the rf-orbitals are defined. 



will be favoured if the energy separation, A, between the t^g and e^ orbitals is 

 small (i.e. a weak ligand field), and the latter by a large energy separation 

 (i.e., a strong ligand field). In the latter case the gain in orbital energy, 

 achieved by having the electrons together in the t^g orbital, is to some extent 

 offset by an increase in the Coulombic repulsion energy, and by a decrease in 

 the quantum-mechanical exchange energy which affords extra stabilization 

 for each pair of electrons with parallel spins. These two effects may be grouped 

 together as 'electron-pairing energy'. Whether a particular complex of a given 

 metal ion has the maximum or minimum number of unpaired electrons 

 depends on the magnitude of this pairing energy and that of the energy 

 separation, A. The magnetic moments of several complexes, notably those 

 of the cobaltic ion, have been discussed from this point of view (Orgel, 1955; 

 Griffith, 1956a), and, following the suggestion of Griffith and Orgel (1957), 

 the two types will be referred to as 'high-spin' and 'low-spin' complexes 

 respectively. 



The co-ordination in haemoprotein compounds is that of an irregular 

 octahedron since the two groups bonded on the z-axis differ from the four 



