The Physical Significance of Bond Valences

In addition to their valence, cations have a second property, namely their coordination number [37]. Some cations are only ever found with one coordination number, e.g. 4 for Be. Others (e.g. Mg) may be found with a range of coordination numbers (4-8), but there will generally be one (6), close to the average (5.98), that is most frequently found [38]. To the extent that the coordination number is a characteristic property of a cation, it is possible to estimate the valence of a bond a priori by dividing the cation s atomic valence by its characteristic coordination number. A set of such values based on the average coordination numbers found in the Inorganic Crystal Structure Database [39] is given in Table 10.1. 

Since anions are more adaptable in their bonding than cations, it is convenient to distinguish between the normal Lewis base strength, calculated by assuming that oxygen is 4-coordinate, and the maximum Lewis base strength, which equals the valence of the strongest bond that the undistorted anion can form. This is usually 

Lewis base strengths correlate well with other measures of basicity such as the p. a [7]. They are given for a variety of anions in Table 10.2 and References [7, 38]. 

The particular virtue of these definitions is that the Lewis acid and the Lewis base strengths are both estimates of the valence of the bond that links the cation with the anion. The most effective bonds will therefore occur between a cation whose Lewis acid strength (Sg) is close to the Lewis base strength (Sb) of the anion. This is known as the Valence Matching Principle. Compounds between badly matched ions, i.e. Be (Sa = 0.5 v.u.) and ClO j (Sb = 0.08 v.u.), are difficult if not impossible to form as both the cation and the anion will be forced into unusual coordination. If they 

The Valence Matching Principle not only allows one to predict chemical stability and solubility, but also suggests ways in which the bonding graph of a compound can be constructed, as described in Section 10.5.1. 

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