Selenium catenates

Although sulfur shows a great tendency toward catenation, selenium and tellurium show much less tendency in this regard. Some structures containing chains and rings of Se and... 

The propensity of sulfur, selenium and tellurium to catenate is illustrated by the formation of an extensive series of polyanions for all three chalcogens. The structures of these polyanions exhibit interesting trends within the series in which the ability of tellurium and, to a lesser extent, selenium to adopt... 

Although selenium and tellurium have a smaller tendency to catenation, they form rings (Se only) and long chains in their elemental forms. None of these chains is branched, because the valence of the element is only 2. 

Catenation is particularly important for sulfur and selenium. A variety of chain and ring structures built from divalent atoms are known for both elements, including most of the common allotropes of the two elements. These structures are readily broken down by nucleophiles, but they also form quite readily. Sulfides and thiols (RSH) are readily oxidized, forming S-S linkages. 

The first two sections of this chapter are the usual ones on (1) the discovery and isolation of the elements and (2) the application of the network to group chemistry. The third section, necessitated by the great ability of sulfur to catenate, concentrates on the allotropes and compounds that involve element-to-element bonds. Next is a short section on the relatively new and potentially useful sulfur nitrides. The reactions and compounds of practical importance in the fourth section include sodium-sulfur batteries, the photoelectric properties of selenium and tellurium, and the most important commercial chemical in the world, sulfuric acid. The selected topic in depth is the production, effects, and possible control of acid rain. 

Table 17.2 shows the formulas of the catenated sulfur oxoacids and one resonance structure for each corresponding anion. Note from the table that not all these acids exist in the free state, but all the anions are well-known. None of these acids or anions have selenium or tellurium analogs in which aU the sulfur atoms are replaced by a heavier congener. 

The allotropes of sulfur in the solid, liquid, and gaseous phases include a wide variety of structures dominated by S-S bonds. Cyclooctasulfur, intertwining superchains, helical chains, rings of various sizes, diatomic molecules, and free atoms characterize this element at various temperatures. Although not to the same extent as those of sulfur, the allotropes of selenium and tellurium have similar catenated chains and rings. Sulfur, selenium, and tellurium also exist in a variety of catenated polycations and polyanions. 

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