Octahedral versus tetrahedral coordination: spinels
المؤلف:
CATHERINE E. HOUSECROFT AND ALAN G. SHARPE
المصدر:
INORGANIC CHEMISTRY
الجزء والصفحة:
2th ed p 587
22-2-2017
1375
Octahedral versus tetrahedral coordination: spinels
Figure 20.26 indicates that, if all other factors are equal, d0, high-spin d5 and d10 ions should have no preference between tetrahedral and octahedral coordination, and that the strongest preference for octahedral coordination should be found for d3 and d8 ions. In practice, other factors do operate. For example, the smaller size of tetrahedral complexes results in higher lattice and solvation energies; thus, although Ni2+ (d8) does not form tetrahedral complexes in aqueous solution, it does so in melts and non-aqueous media.
The distribution of metal ions between tetrahedral and octahedral sites in a spinel can be rationalized in terms of LFSEs. In a normal spinel AIIBIII2O4 the tetrahedral sites are occupied by the A2+ ions and the octahedral sites by B3+ ions: (AII)tet(BIII(oct2O4. In an inverse spinel, the distribution is )BIII)tet(AIIBIII)octO4. For spinel itself, A = Mg, B = Al. If at least one of the cations is from the d-block, the inverse structure is frequently (though by no means always) observed: ZnIIFeIII2O4, FeIICrIII2O4 and MnIIMnIII2O4 are normal spinels while NiIIGaIII2O4, CoIIFeIII2O4 and FeIIFeIII2O4 are inverse spinels. To account for these observations we first note the following:
- the Madelung constants for the spinel and inverse spinel lattices are usually nearly equal;
- the charges on the metal ions are independent of environment (an assumption);
- Δoct values for complexes of M3+ ions are significantly greater than for corresponding complexes of M2+ ions.
Consider compounds with normal spinel structures: in ZnIIFeIII2O4 (d10 and d5), LFSE = 0 for each ion; in FeIICrIII2O4 (d6 and d3), Cr3+ has a much greater LFSE in an octahedral site than does high-spin Fe2+; in MnIIMnIII2O4 (d5 and d4), only Mn3+ has any LFSE and this is greater in an octahedral than a tetrahedral site. Now consider some inverse spinels: in NiIIGaIII2O4, only Ni2+ (d8) has any LFSE and this is greater in an octahedral site; in each of CoIIFeIII2O4 (d7 and d5) and FeIIFeIII2O4 (d6 and d5), LFSE = 0 for Fe3+ and so the preference is for Co2+ and Fe2+ 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. FeIIAlIII2O4 is a normal spinel.
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