Excited states ligand field theory

There are two basic approximate approaches to the bonding in molecules—the valence bond (VB) and molecular orbital (MO) theories. Both are applicable to a study of ground states, though in their more usual approximate forms tend to answer different problems. Only the MO theory is useful for a study of excited states. Ligand-field theory is a simpler application of MO theory to systems consisting of a central atom with a partly filled shell surrounded by closed-shell atoms, ions, or dipoles. 

Color from Transition-Metal Compounds and Impurities. The energy levels of the excited states of the unpaked electrons of transition-metal ions in crystals are controlled by the field of the surrounding cations or cationic groups. Erom a purely ionic point of view, this is explained by the electrostatic interactions of crystal field theory ligand field theory is a more advanced approach also incorporating molecular orbital concepts. 

Reviews have covered applications of ligand field theory to excited states and the photochemistry of complexes, especially those of Crin.130-133... 

The ligand field theory of transition metal photochemistry is based on the idea that the bonding changes in excited electronic states are correlated with ligand photolabilization (11-13). Populating the dz2 or dx2 y2 orbitals increases sigma antibonding in the z and xy directions respectively. 

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. 

Table 6. Energies of excited states of the decamethylmetallocene molecular ions calculated by ligand field theory with limited configuration interaction3 48)...

In the research environment which Ballhausen created at FKI, crystal-field theory played a modest role. Focus was from the very beginning put on molecular orbital theory. In the MO theory, the electronic structure of a molecule or a complex is described by a set of molecular orbitals that extend over the whole region of the molecule or complex. Relative to crystal-field theory, MO theory is a ligand field theory. The ligands are treated on the same footing as the otherwise exclusive metal ion. As to the excited states of a complex, there will be new types besides those that may be described more or less well by crystal-field theory. They will be states essentially localized on the ligands and so-called charge transfer states. The various states and the molecular orbitals may be characterized by their symmetries in a similar way as in crystal-field theory. 

Finally, Duer, Fenton and Gerloch [74] provide an overview of some of the most recent Ligand Field Theory studies as applied to the notion of bent bonding in metal complexes. The data described demonstrate the full capabilities of modem LFT which, for metal complexes of fixed geometry, can now give a unified description of magnetic and excited state properties. 

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