Thermodynamic aspects the Irving-Williams series

The distribution of metal ions between tetrahedral and octahedral sites in a spinel (see Box 12.6) can be rationalized in terms of LFSEs. In a normal spinel A°B 04 the tetrahedral sites are occupied by the A ions and the octahedral sites by ions (A ) (B ) 04. In an inverse spinel, the distribution is (B ) (A B )° 04. For spinel itself, A = Mg, B = Al. If at least one of the cations is from the cf-block, the inverse structure is frequently (though by no means always) observed Zn°Fe2 04, Fe Cr 04 and Mn Mn 04 are normal spinels while Ni Ga 04, Co FeP04 and Fe Fe 04 are inverse spinels. To account for these observations we first note the following  

Consider compounds with normal spinel structures in Zn°Fe2 04 (j and d ), LFSE = 0 for each ion in Fe°Cr2 04 (J and d ), Cr + has a much greater LFSE in an octahedral site than does high-spin Fe 

Mn°MnP04 (J and d ), only Mn has any LFSE and this is greater in an octahedral than a tetrahedral site. Now consider some inverse spinels in Ni Ga2 04, only Ni + d ) has any LFSE and this is greater in an octahedral site in each of Co Fe 04 d and d ) and Fe Fe 04 ( 7 and fi ), LFSE = 0 for Fe and so the preference is for Co and Fe respectively to occupy octahedral sites. While this argument is impressive, we must note that observed structures do not always agree with LFSE expectations, e.g. Fe Al2 O4 is a normal spinel. 

In aqueous solution, water is replaced by other hgands (equation 20.20, and see Table 6.7) and the position of equilibrium will be related to the difference between two LFSEs, since is ligand-dependent. 

This is called the Irving-Williams series and is observed for a wide range of ligands. The trend is a hump that peaks at (d ) and not at Ni (d ) as might be 

The distribution of metal ions between tetrahedral and octahedral sites in a spinel (see Box 13.7) can be rationalized in terms of LFSEs. In a normal spinel A B 04 the tetrahedral sites are occupied by the ions and the 

---

Chapter 20 Thermodynamic aspects the Irving-Williams series 587... 

---