Coordination compound valence bond theory

Note to the student The AP chemistry exam does not emphasize complex ions or coordination compounds. There is nothing on the AP exam that involves the concepts of crystal-field theory, low versus high spin, valence bond theory, or other related areas. If you understand the questions presented here, then you are basically "safe" in this area of the exam. Most high school AP chemistry programs do not focus much on this area of chemistry because of time constraints. 

The model that largely replaced valence bond theory for interpreting the chemistry of coordination compounds was Ihe crystal field theory, first proposed in 1929 by Hans Bethe.11 As originally conceived, it was a model based on a purely electrostatic... 

Chapter 11 Coordination Chemistry Bonding, Spectra, and Magnetism 387 Bonding in Coordination Compounds 391 Valence Bond Theory 391 Crystal Field Theory 394 Molecular Orbital Theory 413 Electronic Spectra of Complexes 433 Magnetic Properties of Complexes 459... 

In the following pages, the valence bond theory and the crystal field theory are described very briefly to set more recent developments in their historical context. The rest of the chapter describes the ligand field theory and the method of angular overlap, which can be used to estimate the orbital energy levels. These two supply the basic approach to bonding in coordination compounds for the remainder of the book. 

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. 

Bonding theories for coordination compounds should be able to account for structural features, colors, and magnetic properties. The earliest accepted theory was the valence bond theory (Chapter 8). It can account for structural and magnetic properties, but it offers no explanation for the wide range of colors of coordination compounds. The crystal field theory gives satisfactory explanations of color as well as of structure and magnetic properties for many coordination compounds. We will therefore discuss only this more successful theory in the remainder of this chapter. 

There are two distinct types of coordination compounds separated from one another by their reactivity, which is due to the nature of the bonding from the metal to the ligand. Complex compounds are bound by coordinate covalent bonds described by valence bond theory (see Chapter 6). Addition salts or double salts, however, are bound according to electrostatic interactions, or ionic bonds. (Turn to Chapter 8 for details on ionic bonds and salts.)... 

The valence bond theory was developed by Professor Linus Pauling, of the California Institute of Technology, and made available in his excellent book. The Nature of the Chemical Bond, published in 1940, 1948, and 1960. Along with the late Marie Curie, Professor Pauling is one of the very few persons to have been awarded two Nobel prizes, the Nobel prize in chemistry in 1954 and the Nobel peace prize in 1962. Pauling s ideas have had an important impact on all areas of chemistry his valence bond theory has aided coordination chemists and has been extensively used. It can account reasonably well for the structure and magnetic properties of metal complexes. Extensions of the theory will account for other properties of coordination compounds such as absorption spectra, but other theories seem to do this more simply. Therefore, in recent years coordination chemists have favored the crystal field, ligand field, and molecular orbital theories. 

Pauling and John C. Slater applied the valence bond theory to coordination compounds, describing the bonding in terms of overlap of an orbital of the ligand containing a lone pair of electrons with a vacant hybridised orbital on the central metal ion. This model gave a satisfactory explanation of both the geometry and... 

However, the valence bond theory approach to transition metals has severe limitations. It fails to account for the absorption spectra and magnetic properties of coordination compounds. These and other properties are more satisfactorily explained by crystal field theory or ligand field theory. 

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