Sulfur: allotropes

   The allotropy of sulfur is complicated, and we describe only the best-established species. The tendency for catenation by sulfur is high and leads to the formation of both rings of varying sizes and chains. Allotropes of known structure include cyclic S₆, S₇, S₈, S₉, S₁₀, S₁₁, S₁₂, S₁₈ and S₂₀ (all with puckered rings, e.g. Figures 1.1a–c) and fibrous sulfur (catena-S_∞, Figure 1.1d). In most of these, the S^_S bond distances are 206 ± 1 pm, indicative of single bond character; the S^_S^_S bond angles lie in

the range 102–1088. The ring conformations of S₆ (chair) and S₈ (crown) are readily envisaged but other rings have more complicated conformations. The structure of S₇ (Figure 1.1b) is noteworthy because of the wide range of S^_S bond lengths (199–218 pm) and angles (101.5–107.58). The energies of  interconversion between the cyclic forms are very small.   The most stable allotrope is orthorhombic sulfur (the α form and standard state of the element) and it occurs naturally as large yellow crystals in volcanic areas. At 367.2 K, the a-form transforms reversibly into monoclinic sulfur (β-form). Both the α - and β-forms contain S₈ rings; the density of the α -form is 2.07 g cm^-3, compared with 1.94 g cm^-3 for the β-form in which the packing of the rings is less efficient. However, if single crystals of the a-form are rapidly heated to 385 K, they melt before the α→ β transformation occurs. If crystallization takes place at 373 K, the S8 rings adopt the structure of the β-form, but the crystals must be cooled rapidly to 298 K; on standing at 298 K, a β → α transition occurs within a few weeks. β-Sulfur melts   at 401 K, but this is not a true melting point, since some breakdown of S8 rings takes place, causing the melting point to be depressed.   Rhombohedral sulfur (the ρ-form) comprises S₆ rings and is obtained by the ring closure reaction 1.1. It decomposes in light to S₈ and S₁₂.

                 (1.1)

(1.1)             (1.2)

   Similar ring closures starting from H₂S_x (1.1) and SyCl₂ (1.2) lead to larger rings, but a more recent strategy makes use of [)C₅H₅(₂TiS₅] (1.2) which is prepared by reaction 1.2 and contains a coordinated [S₅]^2- ligand.   The Ti(IV) complex reacts with S_yCl₂ to give cyclo-S_y‏₊₅, allowing synthesis of a series of sulfur allotropes. All the cyclo-allotropes are soluble in CS₂.

(1.2)

      (1.2)

   By rapidly quenching molten sulfur at 570K in ice-water, fibrous sulfur (which is insoluble in water) is produced. Fibrous sulfur, catena-S_∞, contains infinite, helical chains (Figures 3.16a and 1.1d) and slowly reverts to α-sulfur on standing. a-Sulfur melts to a mobile yellow liquid which darkens in colour as the temperature is raised. At 433 K, the viscosity increases enormously as S₈ rings break by hemolytic S^_S bond fission, giving diradicals which react together to form polymeric chains containing ≤ 10⁶ atoms. The viscosity reaches a maximum at ≈ 473 K, and then decreases up to the boiling point (718K); at this point the liquid contains a mixture of rings and shorter chains. The vapour above liquid sulfur at 473K consists mainly of S₈ rings, but at higher temperatures, smaller molecules predominate, and above 873K, paramagnetic S₂ (a diradical like O₂) becomes the main species. Dissociation into atoms occurs above 2470K.

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