Transition metal cations crystal field effects

Chapter 7 discusses some of the thermodynamic properties of transition metal compounds and minerals that are influenced by crystal field effects. The characteristic double-humped curves in plots of thermodynamic data for suites of transition metal-bearing phases originate from contributions from the crystal field stabilization energy. However, these CFSE s, important as they are for explaining differences between individual cations, make up only a small fraction of the total energy of a transition metal compound. In the absence of spectroscopic data, CFSE s could be evaluated from the double-humped curves of thermodynamic data for isochemical compounds of the first transition series. 

Chemical fractionation of transition metal ions during metamorphism depends on the relative stabilities of the cations in the crystal structures of minerals involved in the crystallization processes. Schwartz (1967) demonstrated that crystal field effects are more important than ionic radii in accounting for distributions of certain transition elements. This conclusion has been confirmed by numerous studies (e.g., Annersten and Ekstrom, 1971 Dupuy et al., 1980 Rosier and Bouge, 1983 Hendricks and Dahl, 1987 Dahlia/., 1993). 

The first-order JT effect is important in complexes of transition metal cations that contain nonuniformly filled degenerate orbitals, if the mechanism is not quenched by spin-orbit (Russell-Saunders) coupling. Thus, the JT effect can be expected with octahedrally coordinated and high spin d cations, and tetrahedrally coordinated and d cations. The low-spin state is not observed in tetrahedral geometry because of the small crystal field splitting. Also, spin-orbit coupling is usually the dominant effect in T states so that the JT effect is not observed with tetrahedrally coordinated d, d , d, and d ions. 

In the alkali metal pseudohalides the contribution of cationic wave functions to the valence band structure can be neglected. The optical absorption spectra can therefore be correlated to transitions involving excited states of the anions. However, one can see solid state effects like the superposition of vibronic structure on the molecular symmetry forbidden transition at 5.39 eV in the crystal spectra of the alkali metal azides (76). In the more complex heavy metal and divalent azides, a whole range of optical transitions can occur both due to crystal field effects and the enhanced contributions from cationic states to the valence band. Detailed spectral measurements on a-PbNe (80), TIN3 (57), AgNs (52), Hg(CNO)2 (72) and AgCNO (72) have been made but the level assignments can at best be described as tentative since band structure calculations on these materials are not available at present. 

Their great abundance points to a very stable crystal structure. Spinels are predominantly ionic. The particular sites occupied by cations are, however, influenced by several other factors, including covalent bonding effects (e.g., Zn in tetrahedral sites) and crystal field stabilisation energies of transition-metal cations. 

Plots of the heat changes against atomic number of the transition metal have been made by a number of workers (e.g.8). The exothermicity of the reaction with ethylenediamine increases from Mn(II) to Cu(II) and then falls at Zn(II) paralleling the trend observed in the heats of hydration of these metal cations (7). With the glycinate complexes crystal-field effects will account for the considerable increase in exothermicity in going from Mn(II) to Ni(II). 

We will discuss the crystal field model here. It assumes that the bonding between metal ions and ligands is essentially ionic. More specifically, it considers the effect of approaching Ugands upon the energies of electronic levels in transition metal cations. We will apply this model to octahedral complexes. Before doing so, it maybe helpful to review the electronic structure of uncomplexed trcuisition metal cations, originally covered in Chapter 6. 

The M—OH bond for transition metal ions is stabilized by the influence of the electrical field associated with the oxygen ions coordinating M+s on the electronic structure of the cation. The term C, is a measure of this crystal field stabilization energy, and the product 0.0029C corrects for this effect. Appropriate values of C are listed in Table I. 

Another factor contributing to the asymmetry and breadth of absorption bands in crystal field spectra of transition metal ions is the dynamic Jahn-Teller effect, particularly for dissolved hexahydrated ions such as [Fe(H20)6]2+ and [Ti(H20)6]3+, which are not subjected to static distortions of a crystal structure. The degeneracies of the excited 5Eg and 2Eg crystal field states of Fe2+ and Ti3+, respectively, are resolved into two levels during the lifetime of the electronic transition. This is too short to induce static distortion of the ligand environment even when the cations occupy regular octahedral sites as in the periclase structure. A dual electronic transition to the resolved energy levels of the Eg excited states causes asymmetry and contributes to the broadened absorption bands in spectra of most Ti(m) and Fe(II) compounds and minerals (cf. figs 3.1,3.2 and 5.2). 

Figure 9.4 Effect of pressure on crystal field splitting parameters for transition metal-bearing periclase and corundum (from Drickamer Frank, 1973 Bums, 1985a). (a) Change of A with pressure for four cations in MgO (b) and (c) (on facing page) pressure variations of A with changes of the unit cell a0 dimension of MgO and A1203.

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

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