From Polyselenides

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... 

Cd-chalcogenides (CdS, CdSe, CdTe) are among the most studied materials as photoelectrodes in a photoelectrochemical cell (PEC) (1,2 /3,4). Interest in such PEC s stems from the fact that, in aqueous polysulfide or polyselenide solutions, a drastic decrease in photocorrosion is observed, as compared to other aqueous solutions, while reasonable conversion efficiencies can be attained. 

The chemistry of polyselenides, polytellurides, and their metal complexes is very well established. Typical structures of polyselenide dianions are shown in Fig. 16.8.5. In these species, the Se-Se bond distances vary from 227 to 236 pm, and the bond angles from 103° to 110°. The tethered monocyclic structure of Se - in the complex Sr(15-C5)2(Se9) has a three-connected Se atom forming two long and one normal Se-Se bonds at 295, 247, and 231 pm (anticlockwise in Fig. 16.8.5, with the longest bond represented by a broken line). The other Se-Se bonds are in the range 227-39 pm. 

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... 

Monomeric polyselenide anions [WQ(Se4)2]2 with Q = Se, S, O were synthesized by the reaction of [WSe4]2 and Se8, SeS2, or Se4(NC5Hi0)2, respectively. The crystal structure of [WS(Se4)2] was determined by X-ray analysis. The starting material [MSe4]2 was prepared from W042- and bis(dimethyloctylsilyl) selenide.126... 

The only relevant Group 3 polynuclear complex, [Y8(/i4—0)(/i3—OH)i2(/r—Se4)4Cl2(DMF)i0] 6DMF is prepared from YC13 and polyselenide in DMF. It has an octanuclear core, supported by oxo, hydroxo, and tetraselenido bridges with no Y—Y bonds. No Sc or actinide chalcogenide clusters are known. Lanthanide/selenido clusters have recently been isolated.14... 

Figure 10.9 A back-wall w-GaAs/aqueous polyselenide photoelectrochemical cell. From Licht and Farouzan (1995).

The results for H2Se(aq) and HSe are selected. The value of AfG°(Se P 298.15 K) is compatible with the corresponding value independently evaluated from redox potentials of mono- and polyselenide ions in Section V.3.4.3, (127.9 1.9) kJ-moP. The finally selected value is chosen as the mean of these values... 

Only one study has been published on the protonation of polyselenide ions. Schoneshofer, Karmann, and Henglein [69SCH/KAR] exposed H2Se(aq) to radiolysis with 1.5 MeV electrons and identified H2Se2 as the end product of a series of radical reactions. The acidity constants of this compound were determined from spectropho-tometric measurements, but as no information coneerning the pH determination is provided, the protonation constants reported, logn, log g = 9.3, are of... 

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. 

The diselenaborane was obtained as an unexpected minor product during the reaction of decaborane(14) with polyselenide ion in aqueous base. The major product was the [BjqHj jSe] ion. It was reported previously that aqueous base degraded BjoHj to form the [B9H14]" ion. Subsequent study proved that polyselenide ion reacted rapidly with [BqHjJ" to form B HsSej in good yield. The procedure for the preparation of this diselenaborane in two steps from decaborane(14) is described here. 

However, very often there are deviations from this linearity due to a shift of the position of the band edges under illumination or as a consequence of the reaction with the redox system In order to prevent corrosion, compound semiconductors have often been employed in a saturated solution of their ionic components like sulfides in sulfide solutions with the redox system polysulfide/sul-fide or selenides in the redox system polyselenide/selenide. However, it was found that even under these conditions the crystalline semiconductors can decompose and are transformed either into a surface with numerous lattice defects or into a different polycrystalline material. 

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. 

Fig. 4. Lattice constants (tetragonal subcell, see text) of rare earth polyselenides MSeg as a function of ionic radius rM> The ionic radii used are from [19], that for Y is from [20].

The phase richest in Se is a polyselenide of variable composition between GdSe 775 and GdSei 862- Its existence was concluded from vapor pressure results in the range Se Gd =1.6 to... 

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