Selenium and tellurium

    Selenium possesses several allotropes. Crystalline, red monoclinic selenium exists in three forms, each containing Se₈ rings with the crown conformation of S₈.   Black selenium consists of larger polymeric rings, and the thermodynamically stable allotrope is grey selenium.  Elemental selenium can be prepared by reaction 1.1. By substituting Ph₃PSe in this reaction by Ph₃PS, rings of composition Se_nS _8-n (n = 1–5) can be produced.

                     (1.1)

   Tellurium has only one crystalline form which is a silverywhite metallic-looking solid. In both grey Se and Te, the atoms form infinite helical chains, the axes of which lie parallel to each other. The red allotropes of Se can be obtained by rapid cooling of molten Se and extraction into CS₂. The photoconductivity of Se  and Te arises because, in the solid, the band gap of 160 kJ mol^-1 is small enough for the influence of visible light to cause the promotion of electrons from the filled bonding MOs to the unoccupied antibonding MOs. Although cyclo-Te₈ is not known as an allotrope of the element, it has been characterized in the salt Cs₃[Te₂₂] which has the composition [Cs‏]³⁺[Te₆^3-][Te₈]₂.  Although less reactive, Se and Te are chemically similar to sulfur. This resemblance extends to the formation of cations such as [Se₄]²⁺‏, [Te₄]²⁺‏, [Se₈]²⁺‏ and  [Te₈]^2‏+. The salt [Se₈][AsF₆]₂ can be made in an analogous manner to [S₈][AsF₆]₂ in liquid SO₂ (equation 15.21), whereas reaction 1.2 is carried out in fluorosulfonic acid.   Recent methods use metal halides (e.g. ReCl₄ and WCl₆) as oxidizing agents, e.g. the formation of [Te₈]^2‏+ (equation 1.3). Reaction 1.4 (in AsF₃ solvent) produces [Te₆]^4+‏, 1.1, which has no S or Se analogue.

                      (1.2)

                           (1.3)

                                    (1.4)

    The structures of [Se₄]²⁺‏, [Te₄]^2‏+ and [Se₈]²⁺‏ mimic those of their S analogues, but [Te₈]²⁺‏ exists in two forms. In [Te₈][ReCl₆], [Te₈]^2‏+ is structurally similar to [S₈]^2+‏ and [Se₈]²⁺‏, but in [Te₈][WCl₆]₂, the cation has the bicyclic structure, i.e. resonance structure 1.2 is dominant.

(1.1)                                       (1.2)