Sulfur or Selenium Rings and Chains

Compounds Contamiiig Sulfur or Selenium Rings or Chains - IR and... 

Weak intramolecular interactions between sulfur or selenium and nitrogen are a recurrent phenomenon in large biomolecules. They may occur in the same residue or between neighbours of a peptide chain. The formation of four- or five-membered rings of the types 15.1 and 15.2, respectively, is most common. A feature that is unique to proteins is the participation of sulfur atoms in bifurcated N S N contacts. 

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... 

The one form of tellurium is silvery-white, semimetallic and isomorphous with gray Se. Like the latter it is virtually insoluble in all liquids except those with which it reacts. Gray Se and Te form a continuous range of solid solutions that appear to contain chains in which Se and Te atoms alternate more or less randomly. With sulfur, selenium can give mixed rings S Se8 n (n = 4-7).9... 

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