Coordination compounds molecular orbital theory

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... 

Coordination Organometalhc Chemistry Principles Electronic Structure of Main-group Compounds Electronic Structure of OrganometaUic Compounds Electronic Structure of Clusters Ligand Field Theory Spectra Molecular Orbital Theory Symmetry Point Groups. 

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

Most transition metals have a number of stable oxidation states that lead to different kinds of chemical bonds and facilitate electron transfer reactions. Molecular orbital theory satisfactorily describes bonding in transition metal compounds and coordination complexes. 

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. 

Ballhausen, C. J., and H. B. Gray (1965), Molecular Orbital Theory, Benjamin, New York. Balzani, V, and V. Carassiti (1970), Photochemistry of Coordination Compounds, Academic Press, London. 

One of the best sources is G. Wilkinson, R. D. Gillard, and J. A. McCleverty, editors. Comprehensive Coordination Chemistry, Pergamon Press, Elmsford, NY, 1987 Vol. 1, Theory and Background, and Vol. 2, Ligands, are particularly useful. Others include the books cited in Chapter 4, which include chapters on coordination compounds. Some older but still useful sources are C. J. Ballhausen, Introduction to Ligand Field Theory, McGraw-Hill, New York, 1962 T. M. Dunn, D. S. McClure, and R. G. Pearson, Crystal Field Theory, Harper Row, New York, 1965 and C. J. Ballhausen and H. B. Gray, Molecular Orbital Theory, W. A. Benjamin, New York, 1965. 

Scientists have long recognized that many of the magnetic properties and colors of transition-metal complexes are related to the presence of d electrons in the metal cation. In this section we consider a model for bonding in transition-metal complexes, crystal-field theory, that accounts for many of the observed properties of these substances. Because the predictions of crystal-field theory are essentially the same as those obtained with more advanced molecular-orbital theories, crystal-field theory is an excellent place to start in considering the electronic structure of coordination compounds. 

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. 

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