Qualitative molecular-orbital theory and its applications

It is certainly reasonable to believe that many of the qualitative structural questions important in mineralogy and in solid-state science in general will have fairly simple qualitative answers. While this may not always be 

Qualitative perturbation theory. We may use a standard second-order perturbation energy expression to determine the small modification in the energy of an orbital m due to interaction with other orbitals n. The interaction matrix element may be approximated as + 

A similar approach has been applied to many other ABj-type molecules in which an atom with valence p orbitals, or even a group of atoms, is bonded to a central atom A. For example, HjSiOSiH, may be interpreted as an 8-valence electron AB2 system with A = O and B = SiH, (Tossed, 1984b). The -SiHj group interacts with O primarily by cr bonding and contributes one valence electron to the central atom. Thus, we would qualitatively expect the Si-O-Si angle to be bent, as observed. The SiHj O orbital that interacts with the O p orbital is the lo2 MO, which would be purely Si 3p in character for a planar SiH, group, and would acquire some Si 3s and H Lv character in a pyramidal geometry. 

For constant B, the B-A-B angle decreases as A becomes less electronegative. 

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