Anionic Chalcogen Rings and Chains

4 ANIONIC CHALCOGEN RINGS AND CHAINS 12.4.1 CatenaSvMva Anions 

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 

Sulfur forms a series of homoatomic dianions catena-S (x = 2-8), which, without exception, have unbranched chain structures in the solid state.The electrochemical reduction of cyclo-Sg in aprotic solvents occurs via an initial two-electron process to produce catenaS P In solution, catena- and other long-chain polysulfides, e.g. catena- and catenaSi, dissociate via an entropy-driven process to give radical anions S (x = 2-4), including the ubiquitous trisulfur radical anion (x = 3). This intensely blue species is the chromophore in the mineral lapis lazuli, which is used in the manufacture of jewellery. 

The long-chain polysulfides are stabilised in the solid state by combination with a large counteranion, e.g. (R = alkyl), [Ph4P], [(Ph3P)2N]+ (PPN ).  

Typically they are prepared by the reaction of cyclo-Sg with a nucleophile in the presence of one of these cations. For example, the reaction of [PPNjSH with cyclo-Sg in ethanol yields the heptasulfide PFN][catena-Sj] 2EtOH. Orange-red crystals of the octasulfide [EGNH]2[catena-S ] are formed on treatment of a solution of cyclo-Sg in formamide with triethylamine and hydrogen sulfide [eqn (12.12)].  

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