Bond main group compounds

Whereas 3c/4e hypervalent interactions (4.77) tend to be relatively uncommon and fragile in main-group compounds (often leading to transition states for nucleophilic displacement reactions, rather than stable equilibrium species), the corresponding interactions in transition-metal coordination compounds are ubiquitous and robust. The far higher prevalence of hypervalent co-bonding in transition-metal chemistry may be attributed to three major factors. 

The method has been confined to main-group compounds presumably because of irregularities expected with unsymmetrical charge distributions in transition metal ions. The noble gas compounds remain outside the scope of the method because of the way in which electronegativity is defined (atom compactness relative to interpolated noble atom compactness). The main weakness of the method when applied to fluorides is in the somewhat arbitrary choice of fluorine bond energies. 

The exceptions to the octet rule described in the previous section, the xenon compounds and the tri-iodide ion, are dealt with by the VSEPR and valence bond theories by assuming that the lowest energy available d orbitals participate in the bonding. This occurs for all main group compounds in which the central atom forms more than four formal covalent bonds, and is collectively known as hypervalence, resulting from the expansion of the valence shell This is referred to in later sections of the book, and the molecular orbital approach is compared with the valence bond theory to show that d orbital participation is unnecessary in some cases. It is essential to note that d orbital participation in bonding of the central atom is dependent upon the symmetry properties of individual compounds and the d orbitals. 

As discussed in detail in Volume 22, it is now known that d orbitals are not involved in bonding in Main Group compounds. In particular, it is now clear that the requirement for... 

Much effort has been devoted to the connection between the heuristic chemical models and the quantum theory of molecular electronic structure in theoretical chemistry in recent years. It is important to understand why and where the models are valid descriptions of experimental results and why and where they fail. It is also possible to examine the validity of the explanations that are given to understand the reactivity, geometry, and other properties of molecules. This article deals with important chemical models and recent quantum chemical studies, which shed light on the electronic structure and bonding situation in main-group compounds. 

Bonding Energetics of OrganometaUic Compounds Electronic Structure of Main-group Compounds Electronic Structure of Solids Electronic Structure of Clusters Ligand Field Theory Spectra Molecular Orbital Theory Photoelectron Spectroscopy of Transition Metal Systems. 

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