Polysulfides and Polyselenides

Not only is H2S2, the sulfur analog of hydrogen peroxide known, but hydrogen polysulfides having as many as eight atoms of sulfur per molecule have also been described. A mixture of such sulfides (represented as H2S where may be 1,2,3,4,5 or 6) results from acidification of a solution of 

---

Related investigations on polysulfide and polyselenide anions with iron carbonyl reveal that there is an equally rich chemistry of sulfides and selenides, but it is substantially different from that of the tellurides. For instance, attempts to obtain lighter analogs of 51 under similar... 

The propensity of tellurium to form intermolecular interactions distinguishes its chemical properties from those of sulfur and selenium for which such secondary bonding is virtually nonexistent (see Ref. 38 and references therein). By contrast to polysulfides and polyselenides, polytelluride anions can exhibit charges that deviate from —2. 

Polysulfides and polyselenides are very similar, but strongly differ from the Te-rich tellurides (section 14). The RX2 composition is usually attributed to these compounds. In many cases, however, nonstoichiometries occur either from a deficiency or an excess of sulfur, and by a deficiency of selenium. It seems, from recent studies, that large homogeneity ranges do not exist, but rather different super-structures corresponding to narrow composition ranges. 

Recently, the crystal structure of stoichiometric La2S4 was described by Dugue et al. (1978). It is a super-structure, with distortion, of the anti-Fe2As type the orthorhombic Pnma cell is a = 2ao, b - 2co, c = Oo. The structure is formed, as for all the polysulfides and polyselenides of the light lanthanides, by alternating sheets... 

Important results and a detailed insight into aqueous chemical deposition processes have been reported and discussed elsewhere for CdSe [248, 249] and ZnS [250, 251] target products. We should note also the work of Davies et al. [252] who described an alternative method for the chemical growth of metal sulfides and selenides on the basis of polysulfide or polyselenide solutions (containing hexa- and tetra-chalcogen anions) formed by the dissolution of sulfur or selenium in hydrazine monohydrate. ... 

Caution. All reactions involving solutions of lithium polysulfides or polyselenides generate compounds that are malodorous and toxic to varying degrees and consequently must be performed in a well-ventilated fume hood. Li[BEt H] can ignite on contact with water, alcohols, or air. This reagent should be handled only under an inert atmosphere. 

Fig. 14 Chemical species coexisting in polysulfide (a), ferrocyanide (b) and polyselenide (c) aqueous electrolytes used in regenerative PECs.

In contrast to the polysulfides and almost all the polyselenides (exception Cs4Sei6 ), a number of polytelluride anions are also known with total negative charges greater than 2, as, for instance, in NaTe with discrete [Tcs]" ions and in 83X022 with polymeric sheet-Uke [Tcg ] ions. These are examples for the striking variety of structural motifs and formula types (x/y = 5/3, 1/1, 2/3, 2/5, 1/3, 1/4, 1/6, 2/13, 1/7 and 3/22) that are known for the alkali metal polytellurides A TCj, and which reflect the propensity of tellurium both to catenate and to participate in hypervalent coordination polyhedra. Whereas discrete anions are observed for xjy > 2/3, polymeric chain and lamellar anionic networks are present in such tellurides when xfy < 2/5. ... 

Complex equilibria among multiple species are typical also of polyselenide and polytelluride solutions. Aqueous polyselenide solutions have been employed as electrolytes in high-elRciency PEC with photoanodes made of gallium arsenide and cadmium chalcogenides (Chap. 5). Unlike polysulfides, the fundamental equilibria constraining the distribution of polyselenide species have not been meticulously characterized [19]. 

Chapter 15 Metal sulfides, polysulfides, polyselenides and polytellurides 447... 

Structurally, the smaller polyselenide and polytelluride ions resemble their polysulfide analogues, e.g. [Teg] has structure... 

Cell 8 [55]. The next cell also uses a Nafion membrane, but makes use of n-CdSe to drive a polysulfide-polyselenide storage couple. Low-output power density is the biggest drawback in this cell. The storage reaction and cell configuration are... 

In this section, we review the characterization of photoetching patterns evolved on the surface of polycrystalUne CdX(X = S, Se), which are one of the most extensively studied semiconductors as photoelectrodes in photoelectrochemical cells, photoconductors, and other optoelectronic usages. Anodic photocurrent flowing through the electrode-electrolyte interface is entirely due to the oxidative dissolution of the semiconductors themselves to form Cd " " ion and elemental chalco-gen, except in certain redox solutions, such as aqueous polysulfide, polyselenide, and alkanolamine solutions. For example, in an aqueous NaCl solution, CdSe is photoanodically decomposed to form Cd + ion and elemental Se. After the photoelectrolysis of a CdSe electrode under potentiostatic conditions and a subsequent... 

---