The structure of finite complex ions

Diatomic ions are necessarily linear, and some of those given in table 10.01 have in fact already been discussed in chapter 8. The cyanide ion [C=N] is often found in free rotation so that many cyanides of strongly electropositive, metals have simple structures. Thus 

The oxides K02, RbOa and Cs02 crystallize with the same structure as CaC2 and contain diatomic 02 ions. The structure of the superoxide ion, however, presents certain problems for it cannot be formulated in terms of simple electron-pair bonds since the number of 

Triatomic anions may be divided into two classes according as to whether their configuration is linear or bent. 

The univalent ions N3 , CNO, CNS , Br3 , I3 , CUCl , BrICI , etc., are all linear, and many of their compounds with univalent metals have structures similar to those of the carbides, with the groups either all parallel to one another or all parallel to a plane but mutually inclined in two directions at right angles. The [Ag(CN)2] group in K[Ag(CN)2] is also linear. 

A bent P02 group is found in ammonium hypophosphite, NH4H2P02, but this is due to the fact that the compound is not an acid salt but a neutral salt in which the radical must be more properly regarded as the tetrahedral PH202 group. This point is considered in more detail later ( 12.18). 

---

Many finite complex ions found in crystals may exist also in aqueous solution (although there are a few, such as the Cf ion, which may not). In a number of cases, the structures of crystals containing complex ions... 

In this scheme a full positive charge of -FI is assumed. In fact, the polarity of the chemical bonds will be lower. The same is true, however, for the classical carbenium ion model. It was shown by Kazansky 316) that surfece protons in zeolites are, in reality, O—H bonds with a finite dipole moment likewise the carbenium ions assumed in isomerization reactions, etc., in acid zeolites are merely transition states of alkoxy groups with a finite dipole moment of the O—C bond. In the same manner the ions in the above reaction scheme should be read as limiting structures of a complex with nonnegligible polarity. 

The greater part of this survey of octahedral structures will be devoted to systems which extend indefinitely in one, two, or three dimensions. However, in contrast to the limited number of types of finite complex built from tetrahedral groups the number of finite molecules and complex ions formed from octahedral units is sufficient to justify a separate note on this family of complexes. In our survey of infinite structures we shall deal systematically with structures in which there is sharing of vertices, edges, faces, or combinations of these elements. We shall not make these subdivisions for finite structures, which will simply be listed in order of increasing numbers of octahedra involved. 

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. 

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. ... 

The chloro-complexes usually contain finite complex anions. None has the perovskite structure of KMgFg. The complex CsAuClg contains equal numbers of square AuCl4 ions and linear AuCl2 ions in addition to Cs+ ions. CS2CUCI4 contains finite CUCI42- ions, but CsCuClg has a double chain (Fig. 228). 

---