From Alkali Metall Tellurides

For example, no less than 12 different formula types are now known for the alkali metal tellurides MxTey [9, 10]. As the total bond order for Te-Te bonds and their opposite secondary Te---Te interactions must remain constant at 1.0, distances di/di will increase in logarithmic dependence on their individual bond order as d3/d4 shorten. Values of d-y/di can range from those of typical single bonds (269-287 pm [10]) to 304 + 9 pm for symmetrical and modestly distorted 3-center 4-electron interactions. 

The selenides and tellurides are similar to the sulfides, being preparable from ammonia solutions of the alkali metals. They are water-soluble yet partially hydrolyzed like the sulfides, but are more susceptible to oxidation back to the element. Not every member of the class M cSe3,/Te3, has been fully investigated, but the many that have promise few surprises see Selenium Inorganic Chemistry and Tellurium Inorganic Chemistry). The polonides are similar, and also have their own article see Polonium Inorganic Chemistry). 

In the sulphides, selenides, tellurides and arsenides, all types of bond, ionic, covalent and metallic occur. The compounds of the alkali metals with sulphur, selenium and tellurium form an ionic lattice with an anti-fluorite structure and the sulphides of the alkaline earth metals form ionic lattices with a sodium chloride structure. If in MgS, GaS, SrS and BaS, the bond is assumed to be entirely ionic, the lattice energies may be calculated from equation 13.18 and from these values the affinity of sulphur for two electrons obtained by the Born-Haber cycle. The values obtained vary from —- 71 to — 80 kcals and if van der Waal s forces are considered, from 83 to -- 102 kcals. 

We may make two generalizations about crystalline metal halides. First, fluorides differ in structure from the other halides of a given metal except in the case of molecular halides (for example, Sbp3 and SbCl3 both crystallize as discrete molecules) and those of the alkali metals, all the halides of which are essentially ionic crystals. In many cases the fluoride of a metal has a 3D structure whereas the chloride, bromide, and iodide form crystals consisting of layer, or sometimes chain, complexes. (For exceptions, particularly fluorides MF3-MF5, see Table 9.9.) Second, many fluorides and oxides of similar formula-type are isostructural, while chlorides, bromides, and iodides often have the same types of structure as sulphides, selenides, and tellurides. The following examples illustrate these points ... 

None of these methods is utterly satisfactory. The use of alkali-metal solutions in liq NH3 allows intercalation from Li to Cs to be covered. It can readily be used to prepare nonstoichiometric phases but the method presents experimental difficulties. Ammonia is often cointercalated, which favors the formation of trigonal prismatic intercalates owing to the preference of the NH3 for this type of site. On the other hand, the solvation of the A ions by NH3 may determine the composition of the final product in relation to the formation of stable complex species, with a definite formulation, between the slabs of the host. The thermal treatment necessary to remove the NHj may also lead to different structures than those formed at RT it certainly plays a role concerning the phase limits. Alkali-metal solutions in liq NH3 are powerful reducing solutions, and in the case of tellurides, or even for some selenides or sulfides, a reduction of the host structure can occur. 

New in this chapter are a number of mixed tellurides. The new tellurides are less sensitive towards hydrolysis than selenides which in turn are more resistant than sulfides. Stability also increases going from germanium to the higher homologues. All alkali metal derivatives are very sensitive to air and moisture. The compounds are listed in Table 6k. Methods of preparation are summarized in the following scheme. 

---