Ligand field theory results

DGCI (double group configuration inteiaction) calculations. " LFT (ligand field theory) results from Dulick et al. (1986). 

From a consideration of the combination of ligand and metal orbitals, it should be apparent that the overlap is much more effective in an octahedral complex (in which orbitals are directed at ligands) than in a tetrahedral complex (where orbitals are directed between ligands). The result is that the energy difference between the e and t2 orbitals in a tetrahedral complex is much smaller than that between the t2g and eg orbitals in an octahedral complex. As we saw when considering the two types of complexes by means of ligand field theory, At is only about half as large as A0 in most cases. 

At an early stage in the development of ligand field theory, it was found that A the splitting parameter for tetrahedral MX4, should be equal to (4/9) of A0, the splitting parameter for octahedral MX6, and experimental data are in good agreement with this prediction. However, this takes no account of the fact that the M—X distance in tetrahedral MX4 is usually some 8—10% shorter than in octahedral MX6. In the pointcharge crystal field model, A is proportional to R-5, so that if the difference in R between MX4 and MX6 is taken into account, we predict (At/A0) to be 0.6—0.7, compared with the experimental value of about 0.5. An AOM treatment (131) leads to better results, since here we find ... 

Many additional studies have since been made. A summary of the results for a number of octahedral complexes is given in Table 10.5. The predictions of ligand field theory are clearly borne out by the results, which show pronounced depopulation of the field-destabilized e g orbitals and increased population of the stabilized t2g(eg, ag) orbitals relative to the distribution in the high-spin spherical atom. 

Concluding this section, one can state that a satisfactory treatment of the Cr + spectra is possible only on the basis of a MO-modified crystal-field approach (ligand-field theory). The results of such a treatment could be interpreted in terms of the covalency of the Cr—O bonds in good correspondence with X-ray and magnetic data. 

In some respects the ligand field theory is closely related, at least qualitatively, to the valence-bond theory described in the preceding sections, and many arguments about the structure of the normal state of a complex or crystal can be carried out in either of the two ways, with essentially the same results.66... 

As a conclusion, Hartree-Fock calculations are seen to be qualitatively compatible with the simple models (ligand field theory for dd-transitions, and the weak coupling model for CT-transitions). However, the ab initio work strongly suggests that the results be situated in a different conceptual framework. 

The results are consistent with the predictions of the ligand field theory and with the observed photochemical reactions. Bond bending distortions are observed in metal-nitrosyl compounds. The photochemical reactions of CotCCO NO and the photohydrogenation catalyzed by RhCPPhjJ NO provide indirect support for the bending. 

Ramasesha et al.216) introduce a coupling between the HS and LS electronic states via lattice strain to take care of the appearance of residual paramagnetism at low temperatures. The resultant mixing of these states is a contradiction to the observations of Mossbauer measurements where the HS and LS states appear separately (slow relaxation limit). Moreover, this mixing cannot occur in the framework of ligand field theory. 

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