Polyselenides conformation

Although the crystal structure of cyclo-Sey is unknown, vibrational spectra indicate a conformation similar to that of cyclo-Sy. This ring conformation is adopted in the structures of the polyselenides [Na(12-crown-4)]2[Se8] (cyclo-Se6, cyclo-Sev) (Figure 12.2) and [Et4N]2[Se5] l/2cyclo-Se6 cyclo-Sev " in which... 

Selenium forms a series of catena dianions Se similar to those described for sulfur, but the chain length is limited to six atoms (x = 6) in simple salts. However, the catenaSQ dianion has been structurally characterised in the black complex [Na(12-crown-4) ]2[Se8 ] (cyclo-Se6,cyclo-Se7). In addition, selenium forms several higher polyselenides (x > 8), in which one or more of the selenium atoms is three- or four-coordinate. The structures of the hyper-valent species Scio , Sen , and Sei6" are especially interesting. The structure of the Seio dianion in the black PPN salt is bicyclic with both six-membered rings in chair conformations (Figure 12.24a). The three-coordinate... 

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