Applying the different effects

The primary constraint on the coordination number is the anion-anion repulsion. Clearly this determines the maximum possible coordination number and, for hard cations, this is normally the coordination number that is observed. However, if the cation is soft or if the maximum possible coordination number gives rise to very small bonding strengths, other coordination numbers may be found as discussed in Section 6.3. A number of examples will illustrate how these various factors work together. 

For the best match to the bonding strength of ( a = 0.50 vu), Zn should display a coordination number of 4 as is found in ZnO (67454). Although Mg + is similar in size and charge to Zn , the same argument does not apply since Mg + is hard and is normally found only with its maximum coordination 

Since water is a constituent in a great many inorganic solids, and is involved in all chemical reactions performed in aqueous solutions, is the most widely distributed of all cations. When present, it usually plays a pivotal role in the chemistry, whether in the solid or the liquid state (Chapter 5). It is therefore important that its behaviour and the origin of its unusual properties be properly understood. 

The crystal chemistry of the H ion is so anomalous that it is usually considered to be qualitatively different from other cations, yet its anomalous properties can be derived in a perfectly rational way by assuming that H is, in principle, no different from other cations except for its small size. H is the only cation where the anion-anion repulsion predicts a maximum regular coordination number of less than 2, as can be seen in Fig. 6.4, where the point for regular two-coordinate H ( = 0.5 vu) lies well to the left of the line. However, as 

An examination of the stereochemistry of the H+ ion is complicated by a number of factors. Because it has no electron core, hydrogen is difficult to locate using X-rays which are scattered by electrons. In earlier structure determinations its presence was often ignored because it made no contribution to the X-ray diffraction pattern and could not therefore be located. Even when H is included in the model, its position can rarely be accurately determined and in any case the centre of its electron density is usually displaced from the nucleus towards the donor anion by around 20 pm. Accurate positions of the H+ nuclei can be found using neutron diffraction which has provided sufficient information to reveal the essential characteristics of hydrogen bond geometries, but in many of the structures determined by X-ray diffraction the positions of the H cations have had to be inferred from the positions of their neighbouring anions. 

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