CFSE

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. 

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. 

The effect of the CFSE is expected to be even more marked in the case of the heavier elements because for them the crystal field splittings are much greater. As a result the +3 state is the most important one for both Rh and Ir and [M(H20)6] are the only simple aquo ions formed by these elements. With rr-acceptor ligands the +1 oxidation state is also well known for Rh and Ir. It is noticeable, however, that the similarity of these two heavier elements is less than is the case earlier in the transition series and, although rhodium resembles iridium more than cobalt, nevertheless there are significant differences. One example is provided by the +4 oxidation state which occurs to an appreciable extent in iridium but not in rhodium. (The ease with which Ir, Ir sometimes occurs... 

Table 26.6 CFSE values for high-spin complexes of 10 ions...

Tetrahedral complexes arc also common, being formed more readily with cobali(II) than with the cation of any other truly transitional element (i.e. excluding Zn ). This is consistent with the CFSEs of the two stereochemistries (Table 26.6). Quantitative comparisons between the values given for CFSE(oct) and CFSE(let) are not possible because of course tbc crystal field splittings, Ao and A, differ. Nor is the CFSE by any means the most important factor in determining the stability of a complex. Nevertheless, where other factors are comparable, it can have a decisive effect and it is apparent that no configuration is more favourable than d to the adoption of a tetrahedral as opposed to... 

However, with the application in the 19, iOs of crystal held theory to transition-metal ehemistry it was realized that CFSEs were unfavourable to the lormation of tetrahedral d complexes, and previous assignments were re-examined. A typical ca.se was Ni(acac)i. which had often been cited as an example of a tetrahedral nickel complex, but which was shown - in I9. I6 to be trimeric and octahedral. The over-zealous were then inclined to regard tetrahedral d" as non-existent until Hrst L.. M. Venanz.i and then N., S. Gill and R. S. Nyholm" demonstrated the existence of discrete tetrahedral species which in some cases were also rather easily prepared. 

A partial explanation, at least, can be provided for the relative abundances and ease of formation of the above stereochemical varieties of Ni complexes. It can be seen from Table 26.6 that the CFSEs of the d configuration, unlike those of the d configuration (e.g. Co ), favour an... 

FIGURE 7.4 (A) Donor CD4+T cell proliferation, as reflected by dilution of the fluorescence associated with CFSE, is not altered by TCDD exposure. Changes in expression ofT cell activation markers (B) CD62L and (C) CD25, are induced by TCDD exposure and are dependent on cell division. Data represent donor CD4+ T cells responding to alloantigen in F1 hosts 48 hours after adoptive transfer. F1 host mice were treated with vehicle or TCDD one day prior to injection of CFSE-labeled donorT cells. Adapted from Funatake et al., 2005. 

The CFSE contribution to lattice energy is almost insignificant for meta- and orthosilicates in which normal and distorted octahedral coordinations are present (Ml and Ml sites, respectively). As shown in table 1.20, the CFSE gap between normal and distorted octahedral fields is in fact only a few kJ/mole. 

Table 1.19 Lattice energy and CFSE for selected spinel compounds. Values of U, E, and from Ottonello (1986). CFSE values calculated with values proposed by McClure (1957). Data in kJ/mole x = degree of inversion.

Table 1.20 CFSE for Ml and M2 sites in some silicates. Data from Burns (1970) and Wood (1974), expressed in kJ/mole.

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. 

For example, the inverse spinel structure of magnetite (see Chap. 2) results from the fact that the CFSE of Fe is greater for octahedral than for tetrahedral coordination, so Fe preferentially occupies octahedral sites. For Fe the CFSE is zero for both octahedral and tetrahedral coordination, so that this ion has no preference for either type of coordination. 

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-... 

The addition of a fifth electron to a weak field complex gives a configuration t e2 and a CFSE of zero. The two electrons in the unfavorable eg level exactly balance the stabilization associated with three in the t2j/ level (Fig. 11.9a). 

---