Cadmium iodide transition

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 ... 

The dependence of the value of k°2s, for reaction (10), on the iodide ion concentration has briefly been investigated. Using cadmium iodide as the source of iodide ion and methanol as the solvent, it was shown2 5 that kobs was independent of iodide ion concentration when the latter was present in two-fold excess or more. In addition, k°2s was also found to be independent of the source of iodide ion. A mechanism involving attack by I3- on the benzylmercuric halide was proposed, possibly through transition states (VI) or (VII)25. 

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... 

Sulphur is less electronegative than oxygen, and in consequence no sulphide of composition AS2 crystallizes with any of the typically ionic structures commonly found among the oxides A02. A number of sulphides have the cadmium iodide layer structure, but many others, particularly those of the transition metals, have structures unrepresented among the compounds so far considered. A few of these are of sufficiently common occurrence to warrant discussion. 

In the Table data for most effective cells are summarized. The most effective semiconductor materials are cadmium chalcogenides, and n-GaAs in polychalcogenide solutions, and transition metal dichalcogenides in iodine-iodide solutions as photoanodes p-InP as a photocathode. 

Interferences caused by transition metal ions have been eliminated or reduced by adding various reagents, such as complexing agents, to the sample solution. The addition of these reagents prevents the reduction or precipitation of the interfering metal ion. The interference caused by cadmium, iron, copper, cobalt, and silver on the determination of arsenic can be eliminated with potassium iodide. An addition of potassium iodide reduces the interference of nickel in the determination of arsenic, that of iron in the determination of antimony, and the influence of copper in the determination of bismuth. 

More importantly, the mobile species are likely to be associated and the nature of these species and degree of association will depend on the anion, cation and salt concentration. The majority of studies on divalent cation polymer electrolytes have involved divalent ions as the salt. These series of salts have an associated problem in that results may not be directly comparable because there is considerable variation in the character of the anion-cation bond. For example, the chlorides of zinc, cadmium and mercury show a sharp transition from ionic to covalent character. Likewise, the tendency oF for example, zinc halides to form complexes, decreases on going from the chloride to the iodide. Mercury halides are too covalent to allow free Hg + ion to form in solution. Thus, HgCl dissociates only slightly to give FlgCL and CL. 

Two distinct classes of promoters have been identified for the reaction simple iodide complexes of zinc, cadmium, mercury, indium and gallium, and carbonyl complexes of tungsten, rhenium, ruthenium and osmium. The promoters exhibit a unique synergy with iodide salts, such as hthium iodide, under low water conditions. Both main group and transition metal salts can influence the equilibria of the iodide species involved. A rate maximum exists under low water conditions and optimization of the process parameters gives acetic acid with a selectivity in excess of 99% based upon methanol. IR spectroscopic studies have shown that the salts abstract iodide from the ionic methyl iridium species and that in the resulting neutral species the migration is 800 times faster [127]. 

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