Cadmium iodide, structure

Figure 15.18 Comparison of the nickel arsenide structure (a) adopted by many monosulfides MS with the cadmium iodide structure (b) adopted by some disulfides MS2. The structures are related simply by removing alternate layers of M from MS to give MS2.

The octahedral radii of the table are applicable to complex ions such as [PtCle]—. The radius sum Pt(IV)—Cl is 2.30 A, and the several reported experimental values for salts of chloroplatinic acid range from 2.26 A to 2.35 A. The radii can also be applied to the sulfides, selen-ides, and tellurides of quadrivalent palladium and platinum (PdS2, etc.), which crystallize with the cadmium iodide structure, consisting of layers of MX octahedra so packed together that each X is common to three octahedral complexes. The average deviation between radius sums and reported distances for these substances is about 0.02 A. 

From the observed values of interatomic distances in complex ions such as [SnCh]—, [PbBr0], and [SeBr ]— and from crystals such as TiS2 with the cadmium iodide structure the octahedral radii given in Table 7-17 have been obtained. These correspond not to cPsp bonds, involving d orbitals of the shell within the valence shell, but to sp d2 orbitals, use being made of the unstable d orbitals of the valence shell itself. 

This brings us to a class of compounds too often overlooked in the discussion of simple ionic compounds the transition metal halides. In general, these compounds (except fluorides) crystallize in structures that are hard to reconcile with the structures of simple ionic compounds seen previously (Figs. 4.1-4.3). For example, consider the cadmium iodide structure (Fig. 7.8). It is true that the cadmium atoms occupy octahedral holes in a hexagonal closest packed structure of iodine atoms, but in a definite layered structure that can be described accurately only in terms of covalent bonding and infinite layer molecules. 

Layered structures are extremely prevalent among transition metal halides. Examples of compounds adopting the cadmium iodide structure or the related cadmium chloride structure (Fig. 7.9) are ... 

It is convenient here to consider the compound disilver monofluoride (or silver subfluoride). This was reported to have an anti-cadmium iodide structure in early work (5), and this has been confirmed more recently (6). From X-ray single-crystal results the unit cell is hexagonal, a = 2.996, c = 5.691 A, space-group C3m, and the Ag—F separation 2.451 A, almost identical with that in silver monofluoride (2.468 A). The structure consists of layers, with a plane of fluorine atoms sandwiched between two planes of silver atoms, giving the fluorine atom a coordination of six silver atoms, the same as in silver monofluoride. The silver atoms have separations of 2.996 and 2.814 A, compared with 2.889 A in the metal itself, and in line with the metallic conductivity of the compound. 

Titanium Diiodide. Titanium diiodide is a black solid (p = 499(0) kg/m3 ) that has the cadmium iodide structure. Titaniums occupy octahedral sites in hexagonally dose-packed iodine layers, where a = 411 pm and c = 682 pm (144). Magnetic studies indicate extensive Ti—Ti bonding. Til2 reacts rapidly with water to form a solution of titanous iodide, Til3. 

Hund 65) has evaluated Mr, of the hexagonal cadmium iodide structure as a function of the axial ratio c/o and u, the z-coordinate of the iodide ion divided by c. The values he obtained are given in Table IV. 

Many AX2 halides, particularly those of the transition metals, show one or other of the closely related cadmium chloride and cadmium iodide structures. Both of these structures are formed by the superposition of a series of composite layers, each of which consists of a sheet of cadmium atoms sandwiched between two sheets of atoms of the halogen. The arrangement of one such layer is shown in fig. 8.08, and it will be seen that a characteristic feature of the structure is the asymmetry of the co-ordination the cadmium atoms are symmetrically surrounded by six halogen atoms at the corners of an octahedron, whereas the three cadmium neighbours of each halogen atom all lie to one side of it. In cadmium iodide the structure as a whole is built up by the superposition of such layers in identical orientation, and the structure can therefore be described in terms of the very simple hexagonal unit cell... 

It is clear that the bonding in the cadmium chloride and cadmium iodide structures cannot be purely ionic, for adjacent atoms in neighbouring layers are of the same kind and the forces between them can be only of the van der Waals type. Indeed, this is also evident from... 

We have already seen that in a limited number of AOH hydroxides the OH group behaves as a negative ion of radius 1 53 A, intermediate between the radii of the F and Cl"" ions, and that the structures of these hydroxides are analogous to those of the corresponding halides. The same is true of certain of the 4(OH)2 hydroxides. None of these hydroxides has any of the symmetrical structures characteristic of truly ionic bonding, but those of Mg, Ca, Mn, Fe, Co, Ni and Cd have the cadmium iodide structure. Some other hydroxides, however, have structures quite different from those of the halides. These structures, and the reason for their abnormal properties, are discussed in 12.10-12.12. 

Fluorite structure (C.N. 8 4) Rutile structure (C.N. 6 3) Silica structures (C.N. 4 2) Cadmium iodide structure Molybdenum sulphide structure Chain structure... 

A closely analogous state of affairs is seen in the systems NiTe2-NiTe and TiTe2-TiTe, in which the AB2 compound has the cadmium iodide structure and the AB compound that of nickel arsenide. In both of these structures the B atoms are arranged as in hexagonal close-packing, but in other respects the relationship between them is precisely the same as that between the cadmium chloride and sodium chloride structures, so that solid solution can take place by the same mechanism. Examples such as this, and many others which could be quoted, emphasize that solid solution is in no sense a satisfactory criterion for isomorphism, and that substances may form solid solution even if their structures are formally quite different. 

A further observation is that these cages can be simply related to the heterocubanes they simply lack one metal vertex per M3O4 block. The trigonal prismatic nickel and cobalt cages have also been related to the cadmium iodide structure. Therefore, it is possible that these many diverse structures for Mn to Ni are beginning to fall into a common, albeit highly complex, pattern based on the ways M3O4 blocks are linked. 

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