Polychalcogen structures

In this chapter, we demonstrated that the restriction of building a compound with only one type of an element is not a restriction at all and a multitude of neutral, cationic as well as anionic polychalcogen structures is currently known. As expected for the more electronegative nonmetal (S) and meta metals (Se, Te), the bonding within these moieties is covalent and a small number of interactions, namely, p2-rap2 lone pair repulsion, n- and n -n bonding as well as p2- cr interactions, are sufficient to rationalize the structures and account for the bond lengths alternations or weak transannular interactions that are often found. 

This kind of an rap2- -racr interaction may lead to a marked bond lengths alternation the extreme structural effect of such an interaction is certainly found for S7, but also other neutral and cationic polychalcogen compounds frequently exhibit this interaction. 

For this larger size of isolated polychalcogen entities only the respective neutral S11-S14 homocycles are known no evidence for other neutral or cationic species in this size has been given. S15 was not structurally characterized.82,83... 

The structure of Te62+ in Te6[MCl6] (M = Zr, Hf)104 is the only structurally characterized polychalcogen compound that contains an E5 homocycle (Figure 39). Its structure and bonding is very closely related to the situation found in Te7[AsF6]2 (shown in, Figure 40a-d). 

The structures of individual polychalcogen cations and theoretically based interpretations of their bonding will now be discussed in order of increasing nuclearity n = 4-19. For more details, the reader is referred to the excellent recent review articles of Beck [19] (structures) and Passmore [13] (bonding and energetics). 

Fig. 2.7-9. Molecular structures of polychalcogen cations with ten or more atoms.

Solvothermal techniques have been extensively developed for the synthesis of metal oxides [149-152]. Unlike many other synthetic techniques, solvothermal synthesis concerns a much milder and softer chemistry conducted at low temperatures. The mild and soft conditions make it possible to leave polychalcogen building-blocks intact while they reorganize themselves to form various new structures, many of which might be promising for applications in catalysis, electronic, magnetic, optical and thermoelectronic devices [153-155]. They also allow the formation and isolation of phases that may not be accessible at higher temperatures because of their metastable nature [156, 157]. 

---