Metallic substances molecular orbital theory

As molecular orbital theory evolved over the years, several variations of Langmuir s definition of isosterism were expressed by others these are discussed in more recent publications [68, 69]. Burger s definition [68] of isosterism encompasses the aspects of the previous definitions and states that isosteres are chemical substances, atoms, or substituents that possess near equal or similar molecular shape and volume, approximately the same distribution of electrons, and which exhibit similar physicochemical properties. A few examples of isosteric atoms and substituents are provided in Figure 4.6. Many more examples are available in the literature [69-72], including metal isosteres [73]. 

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. 

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