Polychalcogenide Anions: Structural Diversity and Ligand Versatility

The structures and ligand binding modes of polychalcogenide anions are reviewed in this article. Participation in hypervalent 3 c ‐4 e bonds and/or weaker secondary np 2 → nσ * bonds leads to a strikingly more diverse homopolytaomic anion chemistry for selenium and tellurium in comparison to sulfur, for which only unbranched chain anions S n 2– with n = 2–9 have been reported. Trapped S 2 · – and S 3 · – radicals are responsible for the characteristic colours of the ultramarine pigments and S 3 · – is the dominant sulfur species at the high temperatures and pressures of deep metamorphic settings. Hypervalent linear TeTe 2 4– , T‐shaped TeTe 3 4– and square‐planar TeTe 4 6– units generate a remarkable variety of structural motifs and connectivity patterns for 1 to 3D polytelluride anions. Electron‐deficient polytelluride sheets are, however, prone to periodic spatial modifications of their charge density (CDWs) with a lower total free energy, whose formation is driven by nesting effects in the electronic band structure causing Fermi surface instabilities. Polychalcogenide ligands augment the typical terminal and bridging coordination modes of monoatomic chalcogenide anions through their ability to participate in chelation, side‐on and combination binding modes. In addition to the electrical conductivity and thermoelectric energy conversion properties of the polytellurides, potential applications of the polychalcogenides range from the manufacture of synthetic zeolite pigments and ion‐exchangeable polysulfide aerogels to NIR‐emissive lanthanide polyselenides with high quantum efficiency.