Trisulfur anion radical

A remarkable example of a persistent radical anion is the semiprecious stone, lapis lazuli, known and appreciated as a pigment since ancient times. The species imparting the blue hue is trisulfur radical anion, S , accompanied by variable fractions of S, which introduces a green tint the sulfur radical anions are incarcerated... 

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. 

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