Complex ions ligand field model

Fe(II) salts cannot readily be explained probably spin-forbidden transitions at = 730 nm are also responsible in this case. The field strength parameters can be calculated using the ligand field model if the influence of the sulfate ions on the ligand field is neglected these are a measure of the stability of the aqua complexes in this series. The values of A = 10 Dq are as follows (in the sequence of the test tubes 1-7) 12,350 cm", 13,900 cm, 7,800 cm , 10,400 cm , 9,300 cm and 12,600 cm . ... 

The ligand-field model is a semi-empirical scheme of calculation that deals Avith the interpretation of the properties of complexes of metal ions with a partly filled /-shell. In this model the experiments are described in terms of empirically determined parameters whose coefficients are exactly calculated. 

Fig. 23. Cjv ligand field model for intrazeolite transition-metal ion complexes (left). Changes of the one-electron energy levels starting from the free ion and switching on fields of spherical, C y and symmetry (right). Reprinted from [32] with permission of Academic Press, Inc... 

Crystal field theory is a simple electrostatic model that treats the transition metal ion and the ligands as point charges. It is very successful in predicting in a very simple way the role of the 3d electrons in determining the properties of complex ions. Crystal field theory ignores the fact that the ligands form coordinate bonds with the transition metal ion via their lone pairs. 

As noted in Section 9.1, there are three closely related theories of the electronic structures of transition metal complexes, all making quite explicit use of the symmetry aspects of the problem but employing different physical models of the interaction of the ion with its surroundings as a basis for computations. These three theories, it will be recalled, are the crystal field, ligand field, and MO theories. There is also the valence bond theory, which makes less explicit use of symmetry but is nevertheless in accord with the essential symmetry requirements of the problem. We shall now briefly outline the crystal field and ligand field treatments and comment on their relationship to the MO theory. 

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