Ligands, molecular orbital theory

Frontier Molecular Orbital theory is closely related to various schemes of qualitative orbital theory where interactions between fragment MOs are considered. Ligand field theory, as commonly used in systems involving coordination to metal atoms, can be considered as a special case where only the d-orbitals on the metal and selected orbitals of the ligands are considered. 

There are two major theories of bonding in d-metal complexes. Crystal field theory was first devised to explain the colors of solids, particularly ruby, which owes its color to Cr3+ ions, and then adapted to individual complexes. Crystal field theory is simple to apply and enables us to make useful predictions with very little labor. However, it does not account for all the properties of complexes. A more sophisticated approach, ligand field theory (Section 16.12), is based on molecular orbital theory. 

The g-values and A values of Table IV reveal that the particular layer silicate has more effect on ESR parameters of adsorbed Cu " - than saturation of exchange sites with different cations such as Na+ and Ca +. Also, the smectites as a group have lower g and higher A values than vermiculite. From the perspective of molecular orbital theory, low g and high A values correspond to more covalent bonds between Cu + and the ligand (19). Thus,... 

Two excellent books which offer useful background material to this chapter are An introduction to ligand fields by B. N. Figgis (published by Interscience Publishers) and Atomic and molecular orbital theory by P. O D. Offenhartz (published by the McGraw-Hill Book Co.). 

For conciseness, the title of this chapter is simply Ligand Field Theory. However, many of the principles which will be developed are as much a part of crystal field theory and the molecular orbital theory of transition metal complexes as they are of ligand field theory. Indeed the three theories are very closely related, and hence it seems advisable to begin this chapter with a brief, historically oriented discussion of the nature of these theories. 

Ligand field theory may be taken to be the subject which attempts to rationalize and account for the physical properties of transition metal complexes in fairly simple-minded ways. It ranges from the simplest approach, crystal field theory, where ligands are represented by point charges, through to elementary forms of molecular orbital theory, where at least some attempt at a quantum mechanical treatment is involved. The aims of ligand field theory can be treated as essentially empirical in nature ab initio and even approximate proper quantum mechanical treatments are not considered to be part of the subject, although the simpler empirical methods may be. 

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