CHAPTER 9.2: Polychalcogenides

Only unbranched chain anions (n = 2–9) are known for sulfur, but participation in hypervalent 3c–4e bonding and/or weaker np²→nσ* secondary bonding leads to a much richer homopolyatomic anion chemistry for selenium and tellurium. For instance, internal interactions cause the cyclization of longer anions with n=9–11 and tri‐, tetra‐, and pentanuclear ligands with central linear or slightly bent hypervalent units have been characterized in a number of metallic phases. Trapped radical anions are responsible for the blue to green colours of ultramarines and is the dominant sulfur species at the higher temperatures and pressures of deep metamorphic settings (T > 250 °C, P >0.5 GPa). Hypervalent linear , T‐shaped , or square‐planar units generate a remarkable variety of structural motifs and formula types for alkali metal polytellurides A_xTe_y, where discrete anions (n = 2, 3) are typical for x/y ≥ 2/3 and polymeric chains or sheets are generally present for x/y ≤ 2/5. Very tellurium‐rich polytellurides with x/y ≤ 1/6 (e.g. RbTe_6, Cs₃Te₂₂, and Cs₄Te₂₈) have been obtained by employing the templating effect of larger alkali metal cations under mild solvothermal conditions. The relatively flat energy hypersurface of electron‐deficient Te–Te–Te interactions in polymeric networks can favour the development of charge density waves (e.g. in K_0.33Ba_0.67AgTe₂), low‐dimensional metals (e.g. UTe₂) and even superconductivity (e.g. for ZrTe₃ at T <2 K). Polychalcogenide anions can exhibit terminal, chelating, bridging, and side‐on coordination modes and combinations thereof.