Polyselenides

Sc2 anions. Highly asymmetric opposite Se4-Se3/Se4-Se5 distances of 242.5(4)/298.9(4) pm are observed in the square-planar coordination sphere of the central Se4 atom. 

Discrete polyselenide chains Se without significant secondary intra- or inter-molecular Se- Se contacts have been structurally characterized for n = 2-8. Crystal structures are known in addition to K5Sc3 for the following binary alkali metal selenides A2Sen in the range n = 2-5 A2Sc2 for A = Na, K, 

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Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

The synthesis, structural characterization and coordination chemistry of polyselenides, Se, and polytellurides, Te t , is a burgeoning field which has sprung into prominence during the past decade. The seminal studies by E. Zintl and his... 

Reaction of Na2Se and Na2Sc2 with Se in the presence of ethanolic solutions of tetraalkyl-ammonium halides and catalytic amounts of I2 yields dark green or black crystalline polyselenides (jc = 3,5-9) depending on the conditions used and the particular cation selected. Tetraphenylphosphonium salts and crown ether complexes of alkali or alkaline earth cations in dimethylformamide solution can also be used. " )... 

Reaction of P4Sc4 with soluble polyselenides afforded the first isolated P/Se anion, the yellow P2Seg (17) which further reacts with Fe(CO)s to generate the novel brown cluster anion [Fe2(CO)4(PSc5)2] (18).Numerous other examples are known indeed, the whole field is still rapidly developing and many new types of compound are being synthesized and characterized each year. 

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]. 

Sharp KW, Koehler WH (1977) Synthesis and characterization of sodium polyselenides in liquid ammonia solution. Inorg Chem 16 2258-2265... 

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. ... 

Kanatzidis and co-workers have obtained polyselenides of varied stoichiometry depending upon reaction conditions. The reaction of AgN03 with Na2Se5 gives different complexes in function of the cation (Scheme 8).1036,1037 A similar compound (Ph4P)2[Ag4(Se4)2(Se5)] has been obtained from Na2Se4 generated in situ."1038... 

The zinc bis(tetraseleno) anion, [Zn(Se4)2]2-, has been crystallized with a number of cations, synthesized by a variety of methods, and its reactivity studied.581-584 The compound (hexaseleno) (tetraseleno)zinc (64) was synthesized from zinc acetate in the presence of lithium polyselenide and structurally characterized, demonstrating seven-membered and flve-membered ZnSen rings.585 Mixed donor neutral monomers have also been formed with imidazole A-donor ligands, ZnSe4 (A-methylimidazole)2.553... 

Only molecular S7 and Se7 are firmly known, yet only the former has been characterized by X-ray analysis. However, Re complexes of Se7 are known55,56 and the neutral Se7 homocycle cocrystallized with salts of polyselenides.57,58 S7+ is thought to be present in S02 solutions of sulfur cations however, this is an unproven but likely hypothesis. Other than that, only the structures of Sig2+ and Se172+ contain E7 homocycles (see there). 

Dhingra, S. Kanatzidis, M. G. 1990. The use of soluble metal-polyselenide complexes as precursors to binary and ternary solid metal selenides. Mater. Res. Soc. Symp. Proc. 180 825-830. 

Although the crystal structure of cyclo-Sey is unknown, vibrational spectra indicate a conformation similar to that of cyclo-Sy. This ring conformation is adopted in the structures of the polyselenides [Na(12-crown-4)]2[Se8] (cyclo-Se6, cyclo-Sev) (Figure 12.2) and [Et4N]2[Se5] l/2cyclo-Se6 cyclo-Sev " in which... 

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... 

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