Racah parameters applications

The second chapter deals with quantum chemical considerations, s, p, d and f orbitals, electronic configurations, Pauli s principle, spin-orbit coupling and levels, energy level diagrams, Hund s mles, Racah parameters, oxidation states, HSAB principle, coordination number, lanthanide contraction, interconfiguration fluctuations. This is followed by a chapter dealing with methods of determination of stability constants, stability constants of complexes, thermodynamic consideration, double-double effect, inclined w plot, applications of stability constant data. 

Ligand-field models serve the purpose of parameterizing experiments ). Their beauty and applicability stem from their derivation from the elementary theory of atomic spectra they are first-order perturbation models based upon a basis set of I functions (assumption 1, p. 71), and hence the interelectronic repulsion within the I shell may be accounted for in terms of Condon and Shortley parameters or Racah parameters. We obtain the expanded radial function model (25, 13,8). 

Ligand field, like crystal field, theory is confined to the role of d orbitals, but unlike the crystal field model, the ligand field approach is not a purely electrostatic model. It is a freely parameterized model, and uses and Racah parameters (to which we return later) which are obtained from electronic spectroscopic (i.e. experimental) data. Most importantly, although (as we showed in the last section) it is possible to approach the bonding in d-block metal complexes by using molecular orbital theory, it is incorrect to state that ligand field theory is simply the application of MO theory. ... 

Virtually all measurements of d—d spectra of heteroatoms have been made on solutions at room temperature (20, 42—44, 50, 114, 183—186), and no transmission data for single crystals or glasses at low temperatures are available. Moreover, some expected transitions are obscured for the reasons explained earlier or because they lie well into the infra-red (e.g. the 4T2 - 4A2 band of CoirWi204o (50)) in a region where absorption by solvent interferes. Therefore, even when the chromophore is known to have symmetry close to O or T, the desirable excess of data over the number of parameters to be ascertained is not always achieved and it may be necessary to assume that the Racah B parameter is invariant for all states of a given complex or that the free ion C/B ratio is applicable. In addition to all this, we are dealing with several structural classes of anions not surprisingly, therefore, we have not found it possible to fit closely our estimates of A and B35 from published frequencies of spin-allowed bands to the Eq. (187)... 

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