Bonding Theories for Coordination Compounds

In Chapters 2 and 3, we considered the history, nomenclature, and structures of coordination compounds. In these earlier discussions, we introduced the metal-ligand (M-L) coordinate-covalent bond in which the ligand shares a pair of electrons with the metal atom or ion. Now we are in a position to consider the nature of the M-L bond in greater detail. Is it primarily an ionic interaction between ligand electrons and a positively charged metal cation Or should the M-L bond be more properly described as predominantly covalent in character Whatever the character of the bond, the description of M-L interactions must account for (1) the stability of transition metal complexes, (2) their electronic and magnetic characteristics, and (3) the variety of striking colors displayed by these compounds. 

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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. 

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... 

Werner s most important contribution, his theory of valency and the structure of coordination compounds, was first presented in 1891 to qualify for a post in the Zurich Polytechnic. He assumed that the valency of an atom, including the carbon atom, is an attractive force emanating from the centre and acting uniformly towards all parts of the surface, rather than directed valency bonds. Although he claimed that this would lead to van t Hoff s configurational formulae, it is on the basis of the latter and directed bonds that Werner s own theory of coordination compounds has been most successfully explained. 

We have seen that the crystal-field model provides a basis for explaining many features of transition-metal complexes. In fact, it can be used to explain many observations in addition to those we have discussed. Many lines of evidence show, however, that the bonding between transition-metal ions and ligands must have some covalent character. Molecular-orbital theory (Sections 9.7 and 9.8) can also be used to describe the bonding in complexes, although the application of molecular-orbital theory to coordination compounds is beyond the scope of our discussion. The crystal-field model, although not entirely accurate in all details, provides an adequate and useful first description of the electronic structure of complexes. 

Alfred Werner. His theory of coordination chemistry was published in 1893 when Werner was 26 years old. In his paper Werner made the revolutionary suggestion that metal ions such as Co3+ could show two different kinds of valences. For the compound Co(NH3)eCI3, Werner postulated a central Co3+ ion joined by "primary valences" (ionic bonds) to three Cl- ions and by "secondary valences"... 

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