Magnetism ligand field theory

The effects of the bonding electrons upon the d electrons is addressed within the subjects we call crystal-field theory (CFT) or ligand-field theory (LFT). They are concerned with the J-electron properties that we observe in spectral and magnetic measurements. This subject will keep us busy for some while. We shall return to the effects of the d electrons on bonding much later, in Chapter 7. 

Aroml G, Brechin EK (2006) Synthesis of 3d Metallic Single-Molecule Magnets. 122 1-67 Atanasov M, Daul CA, Rauzy C (2003) A DFT Based Ligand Field Theory 106 97-125 Atanasov M, see Reinen D (2004) 107 159-178 Atwood DA, see Conley B (2003) 104 181-193... 

The analogy between the two theories is only formal. Crystal field theory is a purely electrostatic approach that does not take into consideration the formation of MOs and the nature of the bond. According to crystal field theory, optical and magnetic properties are ascribed to crystal field splitting between two AOs, whereas in ligand field theory energy splitting occurs between AOs, and... 

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. 

Complexes with geometries near to the trigonal bipyramidai limit show the largest variation of ju with temperature. For these complexes the simple ligand field theory generally fails to reproduce the experimentally observed magnetic moments which are lower than the computed ones. An explanation which has been suggested will be discussed in Section 50.5.1.4.iii. 

Many spectroscope and magnetic studies have been concerned with empirical correlations between these parameters and features of structural and chemical interest in the molecules. It should be noticed, however, that these symmetry-based parameters are global (like /HDvv which is discussed earlier), referring to the field of all ligands as a whole. [The same is true of recent more comprehensive symmetry-defined parameters proposed by Donini et al. (17).] Being based on the minimum assumptions of ligand field theory, and hence, for some, preferred as more basic, these parameters lack possibilities for immediate chemical relevance and appeal. 

Numerous physical properties are studied with transition metal complexes as the subjects, and many of them involve the use of ligand field theory in their interpretation. They vary from those such as the spectra and magnetic properties, which are heavily dependent on ligand field theory, to others such as reaction kinetics where the application of the theory is rather peripheral. 

Obviously, in a presentation such as this, there is not room to develop the basics of the various forms of ligand field theory in detail nor to describe applications to all the relevant physical properties. This chapter will set out to compare the major aspects of the different forms, will give an account of their use in the interpretation of spectra and magnetism of transition metal complexes, and will make some mention of other areas. 

Applications of ligand field theory to many transition metal complexes have played an important role in the interpretation of visihle absorption spectra, magnetism, luminescence, and paramagnetic resonance spectra. 

In this chapter, we have developed the information content of different excited state spectroscopic methods in terms of ligand field theory and the covalency of L—M bonds. Combined with the ground-state methods presented in the following chapters, spectroscopy and magnetism experimentally define the electronic structure of transition metal sites. Calculations supported by these data can provide fundamental insight into the physical properties of inorganic materials and their reactivities in catalysis and electron transfer. The contribution of electronic structure to function has been developed in Ref. 61. 

---