Catenation of sulfur

The catenation of sulfur is discussed in more detail in Section 10.1. 

Speculation about the precise roles of active-site sulfur is tempered by an appreciation of the redox versatihty and interplay of sulfur and molybdenum. This is evident from synthetic systems, where the catenation of sulfur (with attendant redox and/or ligand elaboration) and induced internal electron-transfer reactions are frequently observed. The redox interplay of Mo and S, reflected in undesirable synthetic outcomes, may prove crucial to a fifll description of enzyme behavior. see also Sulfur Inorganic Chemistry)... 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

The characteristic strong tendency of sulfur and its heavier congeners to catenate is reflected in the wide range of polychalcogenide ions, i.e., reduced forms of the elements, that may be discrete in highly ionic salts or dissolved in polar solvents. 

Because of the tendency of sulfur toward catenation, solutions containing sulfides react with sulfur to give polysulfides, which can be represented as SnJ (see Chapter 15). Sulfides of the group IA and IIA metals can also be produced by reducing the sulfates with carbon at high temperature. 

Sulfur is a rather exceptional element for several aspects. It can assume various oxidation states [1] and in these oxidation states it exists in a great number of different chemical forms. Sulfur is a constituent of a large number of industrial products (H2SO4, rubber vulcanization, for instance) and is also at the origin of a major pollutant (SO2) [2]. Sulfur can exist in more than ten allotropic forms at room temperature, that is, in a variety not found with any other element [3-5]. Orthorhombic sulfur is the most stable form at room temperature. It contains crown-shaped Sg molecules, which are stacked in a complex array. It can also be mentioned that the liquid and gaseous phases of sulfur are very complex [6, 7]. Undoubtedly, the existence of catenated species is a general feature of sulfur chemistry and is the basic reason for its complexity. 

This review has shown the complexity of the chemistry and the electrochemistry of sulfur, polysulflde ions, and sulfur cations. This complexity originates from the ability of sulfur to form catenated species, which leads to disproportionation and dissociation equilibria. 

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... 

A sulfur center in oligomeric or polymeric elemental sulfur as well as in sulfur compounds (especially those with catenated sulfur chains) can be attacked by electrophiles and nucleophiles. By far the most important electrophilic attack is the reaction of sulfur with oxygen (also the oxidation of sulfur compounds with oxidation states below -1-6, e.g., H2S or SO2). Despite the complicated and not fully understood mechanism as well as the economic importance (formation of H2SO4), those reactions of sulfur with oxygen are beyond the scope of this section. 

Sulfur shows a marked tendency to react with itself (catenation) and consequently can exist in a large number of acyclic and cyclic Sn species. For the cycles n = 2-20, and n can be >20 for the acyclic chains this behaviour accounts for the complexity of the physical and chemical behaviour of sulfur. 

An important property of sulfur is its ability to form chains of atoms, catenation. The -S-S-links that connect different parts of the chains of amino acids in proteins are an important example. These disulfide links contribute to the shapes of proteins (see Chapter 19). 

As a final example, the anodic response of sulfur electrodes in molten salts can be considerably more complicated even than that of oxygen electrodes, not least because both sulfur atoms and ions have a tendency toward catenation. Hence, it is always necessary to consider families of polynuclear species like and Sx, rather than their dimeric persulfide, 82 and supersulfide, 82 , derivatives. Frequently, it becomes impossible to resolve the electrochemical behavior of these species and spectroscopic methods are required in support of data interpretations. 

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. 

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