Stereochemistry and Bonding in Compounds of Non-transition Elements

The problem to be considered in this Chapter is that of finding relations between the structures of molecules and the nature of the chemical bonds that they contain. This problem has two aspects. The one to which we shall here devote most attention concerns the shapes of the molecules, that is, the angles between the bonds formed by a given atom. The second and shorter part of the discussion will deal with certain aspects of multiple bonding, that is, with certain questions of bond lengths. 

We shall assume implicitly that in two-center bonds the electron density is distributed symmetrically about the internuclear axis. In other words, we shall not explicitly consider the possibility of bent bonds. While this is unquestionably a good and useful approximation—doubtless a much better one than many others usually made in simple valence theory—it is well to remember that, except in special cases where symmetry considerations prohibit it, chemical bonds may well be, at least a little, bent. 

The Valence Shell Electron Pair Repulsion, VSEPR, Model1 

In this model the arrangement of bonds around the central atom is considered to depend upon how many valence-shell electron pairs, each occupying a localized one- or two-center orbital, are present, and on the relative sizes and shapes of these orbitals. The first rule is (1) The preferred arrangement of a given number of electron pairs in the valence shell of an atom is that which maximizes their distance apart. 

Each electron pair is assumed to occupy a reasonably well-defined region of space and other electrons are effectively excluded from this space. Hence, localized electron-pair orbitals behave as if they repel each other and they adopt that arrangement in space that keeps them, on the average, as far apart as possible. For electron pairs in the same valence shell, the arrangements that maximize the least distance apart of any two pairs are listed in Table 4-1. 

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