Sulfur-selenium rings

In summary, it should be pointed out, however, that the exact mechanism of the thermal rearrangement of sulfur rings and chains is still unknown and that further investigations are necessary in this connection. For any further discussion it may also be interesting to take into account the recent results on the molecular rearrangement reactions of cyclic selenium sulfides and of elemental selenium which take place at considerably lower temperatures compared with elemental sulfur and for which — as far as solid selenium is concerned — interesting ionic mechanisms have been proposed... 

Se Sj2 CS2 and CS crystallize in the trigonal space group R3m. The molecular geometry ot he twelve-membered selenium sulfide ring molecule resembles that of Si2. The structure, however, is also disordered. The distribution of sulfur and selenium over the atomic sites is shown in Fig. 2. 

One of the major differences between two-coordinated sulfur and selenium is the tendency of the latter to increase its coordination number, as can be seen from the fairly strong intermolecular interactions between selenium rings in the various crystal structures. The shortest intermolecular distance observed (335 pm in y-Se8) is 65 pm shorter than the van der Waals distance (400 pm) but still 101 pm longer than the single bond distance defined above. In the case of sulfur rings the shortest observed intermolecular contact (323 pm in Si0 (88) is 37 pm shorter than the van der Waals distance (360 pm) but 101 pm longer than the single bond in S8. 

An ab initio MO study of eight-membered sulfur-rich tellurium selenium sulfide rings has shown that the most stable species contain Se Te bonds. This is exemplified by... 

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