Molecular Orbital and Ligand Field Theories

3) This interesting comment comes from C.K. Jorgensen, Modern Aspects of Ligand Field Theory, Elsevier, New York, 1971. 

---

Transition-metal complexes may also be interpreted from the viewpoint of molecular orbital theory, which assumes that the cation and ligand coordinate by overlap of atomic orbitals to form molecular orbital and crystal-field theories predict an arrangement of the same number of electrons in orbitals with energies in the same order, even though they consider the nature of the bonding in the complex from different points of view. 

Ballhausen, C. J and Gray, H. B (1965). Molecular Orbital Theory, Benjamin, New York. Contains a detailed account of crystal and ligand field theories for transition metal ions. 

Molecular Orbital Theory and Ligand Field Theory... 

The application of organometallic compounds in medicine, pharmacy, agriculture and industry requires the accurate determination of these metals as part of their application. Most % complexes characterised by direct carbon-to-carbon metal bonding may be classified as organometallic and the nature and characteristics of the n ligands are similar to those in the coordination metal-ligand complexes. The -complex metals are the least satisfactorily described by crystal field theory (CFT) or valence bond theory (VBT). They are better treated by molecular orbital theory (MOT) and ligand field theory (LFT). There are several uses of metal 7i-complexes and metal catalysed reactions that proceed via substrate metal rc-complex intermediate. Examples of these are the polymerisation of ethylene and the hydration of olefins to form aldehydes as in the Wacker process of air oxidation of ethylene to produce acetaldehyde. 

Theory is a term that is very widely used by chemists. To take the area of chemical bonding as an example, chemists widely refer to molecular orbital (hereafter MO) theory, valence bond (VB) theory, hybridization theory, valence shell electron pair repulsion (VSEPR) theory, and ligand field theory. And even those probably do not exhaust the list. 

Examples where group theory can be applied successfully are molecular orbital theory, molecular spectroscopy, multipole expansions, and ligand field theory. In general, group theoretical results can be used as classification scheme and/or to simplify numerical calculations. 

Some early applications of the method were made by Japanese workers [15], but the 1950s were the heyday of crystal- and ligand-field theory [16]. It was not until the early 1960s that the molecular orbital approach for transition-metal complexes gained momentum. This interest was driven by a combination of spectroscopists ... 

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. 

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. 

---