A polysulfane is a chemical compound of formula H₂S_n, where n > 1 (although disulfane (H₂S₂) is sometimes excluded).^[1][2][3] Compounds containing 2 – 8 sulfur atoms have been isolated, longer chain compounds have been detected, but only in solution.^[4] H₂S₂ is colourless, higher members are yellow with the colour increasing with the sulfur content. In the chemical literature the term polysulfanes is sometimes used for compounds containing −(S)_n−, e.g. organic polysulfanes R¹−(S)_n−R².

Polysulfanes consist of unbranched chains of sulfur atoms terminated with hydrogen atoms. The branched isomer of tetrasulfane H₂S₄, in which the fourth sulfur is bonded to the central sulfur, would be described as trithiosulfurous acid, S=S(−SH)₂. Computations suggests that it is less stable than the linear isomer HS−S−S−SH.^[5] The S-S-S angles approach 90° in trisulfane H₂S₃ and higher polysulfanes.^[4]

Polysulfanes can easily be oxidised, and are thermodynamically unstable with respect to decomposition (disproportionation) readily to H₂S and sulfur:

8 H₂S_n → 8 H₂S + (n − 1) S₈ (in this chemical reaction, S₈ is cyclo-octasulfur, one of the allotropes of sulfur)

This decomposition reaction is catalyzed by alkali. To suppress this behavior, containers for polysulfanes are often pretreated with acid to remove traces of alkali.^[6]

In contrast to the thermodynamic instability of polysulfates, polysulfide anions form spontaneously by treatment of S²⁻ with elemental sulfur:

S²⁻ + (n − 1) S → [S_n]²⁻

Beyond H₂S and H₂S₂, many higher polysulfanes H₂S_n (n = 3 – 8) are known.^[7] They have unbranched sulfur chains. Starting with disulfane H₂S₂, all known polysulfanes are liquids at room temperature. The density, boiling point and viscosity correlate with chain length. Physical properties of polysulfanes are given in the table below.^[6]

They also react with sulfite and cyanide producing thiosulfate and thiocyanate respectively.^[6]

Polysulfanes can be made from polysulfides by pouring a solution of a polysulfide salt into cooled concentrated hydrochloric acid. A mixture of metastable polysulfanes separates as a yellow oil, from which individual compounds may be separated by fractional distillation. Other more selective syntheses are:

Na₂S_n + 2 HCl → 2 NaCl + H₂S_n (n = 4, 5, 6)

S_nCl₂ + 2 H₂S_m → 2 HCl + H₂S_n+2m

The reaction of polysulfanes with sulfur dichloride or disulfur dichloride produces long-chain dichloropolysulfanes:

2 SCl₂ + H₂S_n → 2 HCl + S_2+nCl₂

2 S₂Cl₂ + H₂S_n → 2 HCl + S_4+nCl₂

The reaction with a sulfite salt (a base) quantitatively decomposes the polysulfane to produce thiosulfate and hydrogen sulfide:

(n − 1) SO2−3 + H₂S_n → (n − 1) S₂O2−3 + H₂S