Polyselenides and polytellurides

Although Se and Te analogues of polysulfanes do not extend beyond the poorly characterized H2Se2 and H2Te2, the chemistries of polyselenides, polytellurides and their metal complexes are well established. Equations 16.44—16.47 illustrate preparations of salts of [Se ] and [Te ] see Section 11.8 for details of crown ethers and cryptands. Solvothermal conditions may also be used for the synthesis of [Se ] and [Te ] anions. For example, when CS2CO3 and Se react in superheated MeOH solution, the CS2CO3 facilitates the disproportionation of Se to and 0x0- 

Structurally, the smaller polyselenide and polytelluride (16.44) resemble their polysulfide analogues, e.g. [Tcs] has 

The cyclic [Sg] radical has been prepared by reaction 15.42. In [Ph4P][Sg], the anion adopts a chair conformation, with two S—S bonds significantly longer than the other four (structure 15.18). 

19 with a helically twisted chain. The structures of higher anions are less simple, e.g. [Te ] (15.20) can be considered in terms of [Te4] and [Te3] ligands bound to a Te centre. Similarly, [Sen] can be described in terms of two [865] ligands chelating to an Se + centre. The coordination 

Although 8e and Te analogues of polysulfanes do not extend beyond the poorly characterized H2Se2 and H2Te2, the 

[TeSe2 and [TeSe3 possess bent and trigonal pyramidal structures, respectively. Rationalize these stmctnres in terms of the available valence electrons, and draw a set of resonance structures for (TeSe3). Ensure that each atom obeys the octet rule. 

The Te(Ses)2 ion contains two Se2] bidentate ligands coordinated to a square planar tellurium centre, (i) What is the oxidation state of Te in Te(Ses)2 (ii) Each TeSe -ring has a chair-conformation. Draw the structure of Te(Ses)2, given that the Te atom lies on an inversion centre. 

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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... 

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