Stabilization, crystal field

Although Fc304 is an inverse spinel it will be recalled that Mn304 (pp. 1048-9) is normal. This contrast can be explained on the basis of crystal field stabilization. Manganese(II) and Fe" are both d ions and, when high-spin, have zero CFSE whether octahedral or tetrahedral. On the other hand, Mn" is a d and Fe" a d ion, both of which have greater CFSEs in the octahedral rather than the tetrahedral case. The preference of Mn" for the octahedral sites therefore favours the spinel structure, whereas the preference of Fe" for these octahedral sites favours the inverse structure. 

For cobalt ferrite, CoFe204, the energy effect in the valency reaction Co(II) + Fe(III) —> Co(III) -H Fe(II) has been calculated to be 1.35 eV from the ionization potentials and crystal-field stabilization effects. The combination Co(II) + Fe(III) is the ground state. For titanium in Fe203 the following reaction is of importance... 

Multipole analysis with high-resolution X-ray data for [Ni(thmbtacn)]2+ was carried out to determine the electron configuration in the C3 symmetry-adapted orbitals of the Ni ion, confirming a higher occupancy of the crystal field-stabilized t2g orbitals relative to the destabilized eg orbitals. This is interpreted in terms of a predominantly ionic metal-ligand interaction.1424... 

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

DR. PATEL In the early stages of some of this work, due to calculations on crystal field stabilization energies, it was thought that vanadium(III) and titanium(III) could be very good candidates for an associative mechanism. Now, there has been some work done on these systems, having to do with enthalpies of activation, which seems to substantiate an associative mechanism. 

Table 1.17 Electronic configuration and crystal field stabilization energies for 3d electrons in transition elements. (Ar) = argon core s 2s 2p 3s 3p. ...

Table 1.18 Crystal field stabilization energies for 3d transition elements according to McClure (1957) (1) and Dunitz and Orgel (1957) (2). ...

Ecfs = crystal field stabilization energy. The other column heads are defined as in table 5.12. Data in kJ/mole. 

The populations of the crystal-field-stabilized dxz, dyz, and dxy orbitals are in very good agreement with results of an Extended Hiickel (EH) calculation (third column of Table 10.7), which is perhaps unexpected, given the approximate nature of the calculation. Reasonable agreement is also obtained for dz2, but not for the destabilized dx2 y2 orbital, for which the EH method overestimates the population... 

The cations in these compounds are Fe and/or Fe". In iron oxides, Fe " is always in the high spin (unpaired d electrons) state. As Fe with five d electrons has no crystal field stabilization energy (CFSE see Chap. 6), regardless of whether it is octa-hedrally or tetrahedrally coordinated, there is little preference for one or the other type of site. For Fe , on the other hand, CFSE is higher for octahedral than for tetrahedral coordination, so the octahedral coordination is favoured. 

The M-ferrihydrite coprecipitate contains M-O/OH-Fe and M-O/OH-M as well as Fe-O/OH-Fe linkages. The transition elements stabilize ferrihydrite in the order, Mn < Ni < Co < Cu < Zn (Cornell, 1988 Giovanoli Cornell, 1992). This order does not correspond with that of the electronegativities or the crystal field stabilization energies (CFSE) of these elements, nor does it match the order of binding constants for the M-surface complexes. If Zn is omitted from the series, however, there is a reasonable cor-... 

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