High-spin configurations octahedral coordination

Figure 2.3 Ionic radii for ions commonly found in solids (a) graphical representation (b) periodic table. Note a superscript, indicates a high-spin configuration (Section S4.5) cation radii are those for ions octahedrally coordinated to oxygen, except where marked with a t, which are for ions in tetrahedral coordination...

The high-spin d configuration of Fe , like that of Mn , confers no advantage by virtue of CFSE (p. 1131) on any particular stereochemistry. Some examples of its consequent ability to adopt stereochemistries other than octahedral have just been mentioned and further examples are given in Table 25.3 (p. 1078). These cover the range of coordination numbers from 3 to 8. 

At Xu = 0 Fe was found to be of intermediate stability in octahedral coordination. Unfortunately the valences of the Fe ions could not be clearly determined using the spin integration method of section 5 because the net spin on the Fe did not match a high-spin or low-spin configuration consistent with the average formal oxidation state (+4). This suggests that the d-orbital filling takes on an... 

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