Section 7. Sulfur, Selenium, Tellurium

Hypervalent iodine(III) compounds have found wide application for the oxidation of organic derivatives of nitrogen, sulfur, selenium, tellurium and other elements. Reactions of X -iodanes with organonitrogen compounds leading to the electron-deficient nitrenium intermediates and followed by cyclizations and rearrangements (e.g., Hofmann rearrangement) are discussed in Section 3.1.13. Several other examples of oxidations at a nitrogen center are shown below in Schemes 3.168-3.170. 

The other group 6A elements are sulfur, selenium, tellurium, and polonium. In this section we win survey the properties of the group as a whole and then examine the chemistry of sulfur, selenium, and tellurium. We will not say much about polonium, which has no stable isotopes and is found only in minute quantities in radium-containing minerals. 

In the first section of this chapter some of the properties of the elements hydrogen, carbon, nitrogen, phosphorus, arsenic, antimony, bismuth, oxygen, sulfur, selenium, tellurium, fluorine, chlorine, bromine, and iodine are described. The following sections are devoted to some of their compounds with one another, especially the single-bonded normal-valence compounds. Compounds of nonmetals with oxygen are discussed in the following chapter. 

A Compare the structures of the dioxides of sulfur, selenium, tellurium, and polonium. How do they relate to the metallic or nonmetallic character of these elements (See Section 5 -9.)... 

The chalcogenides are binary compounds of a chalcogen (i.e., the elements of Group Ilb zinc, cadmium, mercury) with a less electropositive element, such as those of Group VIb (oxygen, sulfur, selenium, and tellurium). This section covers the sulfides, selenides, andtellurides. Oxides are reviewed above in Ch. 11. Most of the chalcogenides have useful optical characteristics and their applications are usually found in optics. 

Secondary phosphine chalcogenides (15a) can be readily prepared from oxidation of secondary phosphines (R2PH) with elemental sulfur, selenium or tellurium, although care must be taken to avoid over-oxidation to give the dichalcogenophosphinic acids (see Section 5.2.5). 

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

This section of the coverage of palladium(II) coordination chemistry will concentrate on complexes of the more common, usually unidentate ligands containing sulfur, selenium or tellurium donor atoms coverage of the vast number of chelate complexes of bi- or multi-dentate ligands will be limited but the reader will find references to most classes of palladium(II) chelate complexes and these will provide an entry into the literature. 

Sulfur, selenium, and tellurium form a variety of oxyacids. Certain of these compounds and their derivatives are of great importance, with the most important of all these compounds being sulfuric acid. Because of the extraordinary significance of that compound, it will be described in more detail in a separate section. The major oxyacids of sulfur are shown in Table 15.6. Differences between the sulfur oxyacids and those of selenium and tellurium will be discussed later. 

Until 1980, The Royal Society of Chemistry (previously the Chemical Society) published a section on benzisothiazoles in Specialist Periodical Reports in the volumes on the Organic Chemistry of Sulfur, Selenium, and Tellurium 4 it was then transferred to Heterocyclic Chemistry.5... 

Sulfur, selenium, and tellurium can all be mined from the earth. Large underground deposits are the principal source of elemental sulfur ( FIGURE 22.17). Sulfur also occurs widely as sulfide (S ) and sulfate (S04 ) minerals. Its presence as a minor component of coal and petroleum poses a major problem. Combustion of these unclean fuels leads to serious pollution by sulfur oxides. — (Section 18.2) Much effort has been directed at removing this sulfur, and these efforts have increased the availability of sulfur. The sale of this sulfur helps to partially offset the costs of the desulfurizing processes and equipment. 

The Ti -chalcoacyl derivatives of the previous section are also accessible by direct reaction between the carbyne complex and elemental sulfur, selenium, or tellurium, once again emphasising the metalla-alkyne nature of these carbyne complexes. 

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