Ligand field splitting determination

J 6 Determine the ligand field splitting from the wavelength of light absorbed by a complex (Example 16.3). 

Equation (18.17) predicts values for A0 that are in reasonably good agreement with the values determined by more robust methods. In many instances, an approximate value for the ligand field splitting is all that is required, and this approach gives a useful approximation for A0 rapidly with a minimum of effort. 

Here we discuss two topics the relationship between octahedral and tetrahedral ligand field splitting parameters, and the pressure-dependence of the octahedral parameter a common feature of these is the use of the AOM to determine the variation of orbital splittings with the intemuclear distance. 

The single d-electron of an octahedral [Ti(H20)6]3+ complex can be excited from the 2g-orbital into one of the eg-orbitals if it absorbs a photon of energy equal to A0 (Fig. 16.34). We see that we can use the wavelength of the electromagnetic radiation absorbed by a complex to determine the ligand field splitting. 

For the pseudo-tetragonally elongated Ml octahedral site in the olivine structure, an alternative method for obtaining the CFSE of Fe2+ ions in this site is to evaluate the ligand field splitting parameters in the equatorial plane and along the axial direction using eqs (3.14) to (3.16). The crystal field spectral data for the fayalite Ml site (see fig. 5.11a) yield values for 10 Dq(m) and 10 Dq(eq) of 8,172 cm-1 and 9,327 cm-1, respectively. The mean 10 Dq value of 8,942 cm-1 and the CFSE of 4,133 cm-1 compare favourably with A0 = 8,830 cm-1 and CFSE = 4,250 cm-1 for Fe2+ in the fayalite Ml sites determined by the baricentric method (fig. 5.11a). 

However, it is still necessary to understand what factors determine the magnitude of the ligand field splitting energy so that predictions can be made as to the nature of complexes formed between specific metal ions and ligands. As described earlier, the magnetic moments of the complexes can be used to determine how the electrons are arranged because one case has four unpaired electrons but the other has only two. 

Axial and rhombic zero-field splitting parameters of Kramers doublet ground states and ligand field splitting parameters of non-Kramers doublet degenerate ground states can be determined from the variable temperature variable field data. 

However, it remains to be determined whether the azido group is already coordinating the iron(ni), since the ligand field splitting is smaller compared to that of the resting azido complex. The other possibility is that the azide is not coordinating the iron but perturbs interactions between the iron-aqua ligand and its distal-pocket environment. 

The extent of mixing of different spin-orbital states expressed by the angle (x) is determined by the ratio of the trigonal ligand field splitting parameter A (positive if [Pg.193]

Absorption spectra of coordination compounds can be used to determine the magnitude of the ligand field splitting, which is A for octahedral complexes. It should be made clear from the outset that the accuracy with which Ag can be determined is to some extent limited by the mathematical tools used to solve the problem. Absorption spectra often have overlapping bands to determine the positions of the bands accurately, therefore, requires an propriate mathematical technique for reducing overlapping bands into their individual components. Such analysis is beyond the scope of this text. However, we can often obtain A values (and sometimes values of the Racah parameter, B) of reasonable accuracy simply by using the positions of the absorption maxima taken directly from the spectra. 

The ligand field splitting A can be determined from the electronic spectrum of a given complex, but, in most cases, it is not related simply to the color of its solution. 

Table 19-1. DFT values for energies/eV of all 45 Slater determinants (SD) due to the d configuration of Fe in tetrahedral Fe04 in terms of interelectronic repulsion B and C and the cubic ligand-field splitting parameter 0Dq... 

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