Crystal field stabilisation energy

We may use exactly similar arguments to obtain total CFSE terms for the various d electron configurations within a tetrahedral crystal field. It is quite possible to construct crystal field splitting diagrams for any of the other geometries commonly adopted in transition-metal complexes, and to calculate the appropriate CFSE terms. 

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As a consequence, the crystal field stabilisation energy (CFSE) for grafted metal complexes will be less negative than for the original, homogeneous-phase complex, if one starts with aqua complexes (which is most usual in catalyst preparation) or with ammine complexes. Since the change in CFSE is a major component of the adsorption enthalpy AHads- transition metal adsorption is not expected to be strongly exothermic. 

The conditional stability constants (log K<.) obtained for copper with humic compounds extracted from soils and natural waters are invariably greater than those for other transition metals (see Table IV). This is expected from the enhanced levels of crystal field stabilisation energy which result fi-om the splitting of the 3d electronic orbitals on Cu by an octahedral field (Mackay and Mackay, 1969). The divei ence in the values of log Kc shown in Table IV, may, in part, have arisen from intrinsic variations in the copper-binding properties of the various humic samples. However, these deviations may also be explained in terms of the different experimental conditions employed (pH, ionic strength, temperature, for example) and the assumptions made in the calculations. For example, an increase in the pH will enhance the availability of dissociated binding sites (see Section 6) which are then free to participate in further complexation of copper and... 

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. 

Table 2.3. Crystal field stabilisation energies for transition-metal cations on octahedral and tetrahedral spinel sites. 

Several quantitative studies (Dunitz Orgel, 1957 McClure, 1957) have allowed a direct evaluation of the importance of this contribution to the cation distributions in spinels. By combining spectroscopic and magnetic data on a variety of compounds, an estimate of the octahedral site preference energy was obtained. This is the difference in crystal field stabilisation energy between octahedral and tetrahedral sites, and is given in Table 2.3. 

It turns out that crystal field theory accounts adequately for practically all the experimental results in spinels. The fact that all the known chromites have a normal distribution is consistent with the high octahedral field stabilisation energy value calculated for Cr (S = 3/2). In ferrites, the arrangement is very dependent on the divalent cation, since Fe has no crystal field stabilisation energy. When the divalent cation also shows no clear preference, ferrites with 3 values between zero and one (mixed ferrites) are obtained. 

The inert metal complexes such as Cr(III) (CFSE = -1.2A) and Co(III) (CFSE = -2.4A) have large crystal field stabilisation energies. In the case of Co(lII) with six nitrogen donors the CFSE is ca. 250 kJ mof. Energies of this magnitude compare with the values of AH for ligand exchange processes, thus for the reaction. 

Note the crystal field stabilisation energy of the square pyramid in a d system is always greater than for that of the trigonal bipyramid. 

In Fig. 1 the relative metal-chlorine and metal-bromine bond strengths are plotted against the atomic number of the metal. The filled-in circles and squares represent the values corrected for the crystal field stabilisation energy. 

Solvent exchange at transition-metal ions in aqueous solution. Rate constants, activation enthalpies, and crystal-field stabilisation energies. 

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