Hydrates of oxy-salts, hydroxides, and halides

We exclude from the major part of our discussion the structures of hydrated complex salts, since the principles determining their structure are much less simple. For example, in hydrated complex halides A (BX ). PH2O, which we consider briefly later, the water is in some cases attached to B (e.g. in (NH4)2(VFs. H2O)) while in others (e.g. K2(MnFj). H2O, p. 383) it is situated together with the A ions between BX complexes (here infinite octahedral chain ions). Similarly there are octahedral (HgCU) chains in K2HgCl4. H2O between which lie the K ions and the H2O molecules. 

We have seen that the structures of the ice polymorphs and of the ice-like hydrates indicate that the H2O molecule behaves as if there is a tetrahedral distribution of two positive and two negative regions of charge. The arrangement of nearest neighbours of water molecules in many crystalline hydrates is consistent with this tetrahedral character of the water molecule. In hydrated oxy-salts we commonly find a water molecule attached on the one side to two 0 atoms of oxy-ions and on the other to two ions or to one ion thus  

Some of the numerous possible environments of a water molecule in hydrates are shown in Fig. 15.10. It might seem logical to classify hydrates according to the way in which the water molecules are bonded together. If all the H2O molecules in a particular hydrate have environments of one of the types shown in Fig. 15.10, with 4, 3, 2, 1, or zero H2O molecules as nearest neighbours then the systems of linked H2O molecules (aquo-complex) would be as follows (a) and (b), all possible 

Environment of water molecules in crystals. The larger shaded circles represent M,  

A detailed description of the bonding in hydrates evidently requires a knowledge of the positions of the H atoms. In the earlier X-ray studies it was not possible to locate these atoms, and it was assumed that they were responsible for certain unusually short 0-0 or 0-X distances in the crystals. Later studies, particularly n.d. and n.m.r., have confirmed this and led to the precise location of the H atoms. It is now becoming possible to discuss not only the gross structures of the compounds, that is, the spatial arrangement of the heavier atoms, but also two aspects of the finer structure, namely, the positions of the H atoms in hydrogen bonds and the ordering of the protons. Reference to these topics will be made later. 

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