Oxy-ions

On application of a field across the cell, the anions (assumed in the diagram to form part of siliceous oxy ions constituting the soil) stay put, but the M+ ions are pulled by the electric field toward the cathode (i.e., they are separated from the soil to which they formerly adhered). The degree of removal will be influenced by the solubility of the M+ ions, and the type of complexation (if any) that may occur when they enter the solution. Remaining with the simplified picture of Fig. 15.30, one can see two ways... 

Rule 15. Coordinated oxide ions. Since oxide ions are best regarded as occupying specific coordination positions, they should be designated in the name rather than to use -yl to designate oxy ions as centers in coordination compounds. 

Finite complex ions include the numerous oxy-ions and complex halide ions, the aquo-ions in some hydrates (e.g. A1(H2 0)6 ), and all finite charged coordination complexes, in addition to the very simple ions such as CN , C2 , O2, O , and many others. Crystals containing the smaller or more symmetrical complex ions... 

The same principles determine the structures of the oxides and oxy-ions of these elements in their highest oxidation states, the structural units being tetrahedral MO4 groups ... 

Certain elements, notably V, Nb, Ta, Mo, and W, form complex oxy-ions built from larger numbers of octahedral coordination groups. Examples are included in Table 5.3. In the heteropolyacid ions such as FWJ2O40 and 2 180 2 P atoms occupy the tetrahedral holes at the centres of the complexes. These more complex oxy-ions are described in Chapter 11. 

The 12-electron group includes NO , O2, SO, and S2, and the 14-electron group, the molecules of the halogens and interhalogen compounds XX, and halogen oxy-ions XO . There are few examples of the 11- and 13-electron groups ... 

The simplest systems containing ionic bonds are the gaseous molecules of alkali halides and oxides, the structures of which are noted in Chapters 9 and 12 we refer later to the halide molecules in connection with polarization. The importance of the ionic bond lies in the fact that it is responsible for the existence at ordinary temperatures, as stable solids, of numerous metallic oxides and halides (both simple and complex), of some sulphides and nitrides, and also of the very numerous crystalline compounds containing complex ions, particularly oxy-ions, which may be finite (CO3 , NO3, SOl", etc.) or infinite in one, two, or three dimensions. 

We have taken here as our example a tetrahedral oxy-ion the same problem arises, of course, for other oxy-ions XO3, XO, etc. The figures in the self-explanatory Table 7.15 are simply the numbers of M-0 bonds required to balance the charge on 0 of the oxy-ion non-integral values, which would be mean values for non-equivalent 0 atoms, are omitted. It seems likely that structures corresponding to entries below and to the right of the stepped lines are geometrically impossible. If we wish to apply this information to particular compounds we have to assume reasonable... 

In the stability of their oxy-acids there are great differences between the halogens. The existence of only one oxy-acid of fluorine has yet been firmly established, and the only types of oxy-ion formed by all three halogens Cl, Br, and I are XO", XO3, and XO4 (Table 9.7). Structural studies have been made of HOCl, HOBr, 0-HI03, HCIO4. H2O, HjIOe, and HI3O8. 

X-ray crystallographic examination of salts has established the structures of many mononuclear oxy-ions. There is a small number of polynuclear oxy-ions in which X... 

Another type of more complex finite oxy-ion is formed by the bonding of XO ions to a metal atom, either through X or through one or more of the 0 atoms. Examples are noted in other chapters and include [Co(N02)6] and [Co(N03)4]"-. 

There is much experimental evidence for the formation of complex oxy-ions in solutions of vanadates, niobates, and tantalates. We describe in Chapters 12 and 13 the structures of some crystalline vanadates here we note only certain finite complex ions which exist both in solution and in crystalline salts. The ion of Fig. 11.3(a) has been shown to exist in the salts Na7H(Nb60i9). 15 H20. Light scattering from an aqueous solution of Kg(Ta60i9). I6H2O indicates that the anion species contains 6 Ta atoms and is presumably similar to the ion in the crystal. ... 

As far as is known the oxygen chemistry of Cr is based exclusively on tetrahedral coordination. Cr is present in some, if not all, of the black oxides intermediate in composition between CrOj and Cr03 (p. 947) which are obviously metallic oxides more akin to those of Mo and W. Not much is known of the structural chemistry of oxy-compounds of Mn apart from the fact that the permanganate ion MnOJ is tetrahedral. We have already referred to MujOt. Both Mo and W form tetrahedral oxy-ions but their complex oxy-chemistry is largely based on octahedral coordination, which appears even in the pyro -salts Na2Mo207 and Na2W207 (p. 431), Some finite oxy-ions of V, Nb, Mo, and W are described in... 

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