Sulfur monoxide

Production and reactions

The SO molecule is thermodynamically unstable, converting initially to S₂O₂. Consequently controlled syntheses typically do not detect the presence of SO proper, but instead the reaction of a chemical trap or the terminal decomposition products of S₂O₂ (sulfur and sulfur dioxide).

Production of SO as a reagent in organic syntheses has centred on using compounds that "extrude" SO. Examples include the decomposition of the relatively simple molecule ethylene episulfoxide:

C₂H₄SO → C₂H₄ + SO

Yields directly from an episulfoxide are poor, and improve only moderately when the carbons are sterically shielded. A much better approach decomposes a diaryl cyclic trisulfide oxide, C₁₀H₆S₃O, produced from naphthalene-1,8-dithiol and thionyl chloride.

SO inserts into alkenes and alkynes to produce thiirane oxides and thiirene S-oxides respectively. It reacts with dienes to produce 2,5-dihydrothiophene S-oxides.

Sulfur monoxide may form transiently during the metallic reduction of thionyl bromide.

Generation under extreme conditions

In the laboratory, sulfur monoxide can be produced by treating sulfur dioxide with sulfur vapor in a glow discharge. It has been detected in single-bubble sonoluminescence of concentrated sulfuric acid containing some dissolved noble gas.

Benner and Stedman developed a chemiluminescence detector for sulfur via the reaction between sulfur monoxide and ozone:

SO + O₃ → SO₂* + O₂

SO₂* → SO₂ + hν

(* indicates an excited state)

Occurrence

Ligand for transition metals

Transition metal complexes of sulfur monoxide are well-known. One example is Fe₃(μ³-S)(μ³-SO)(CO)₉.

Astrochemistry

Sulfur monoxide has been detected around Io, one of Jupiter's moons, both in the atmosphere and in the plasma torus. It has also been found in the atmosphere of Venus, in Comet Hale–Bopp, in 67P/Churyumov–Gerasimenko, and in the interstellar medium.

On Io, SO is thought to be produced both by volcanic and photochemical routes. The principal photochemical reactions are proposed as follows:

O + S₂ → S + SO

SO₂ → SO + O

Sulfur monoxide has been found in NML Cygni.

Biological chemistry

Sulfur monoxide may have some biological activity. The formation of transient SO in the coronary artery of pigs has been inferred from the reaction products, carbonyl sulfide and sulfur dioxide.

Sulfur monoxide dication

Sulfur dioxide SO₂ in presence of hexamethylbenzene C₆(CH₃)₆ can be protonated under superacidic conditions (HF·AsF₅) to give the non-rigid π-complex C₆(CH₃)₆SO²⁺. The SO²⁺ moiety can essentially move barrierless over the benzene ring. The S−O bond length is 142.4(2) pm.

C₆(CH₃)₆ + SO₂ + 3 HF·AsF₅ → [C₆(CH₃)₆SO][AsF₆]₂ + [H₃O][AsF₆]

Disulfur dioxide

SO converts to disulfur dioxide (S₂O₂). Disulfur dioxide is a planar molecule with C_2v symmetry. The S−O bond length is 145.8 pm, shorter than in the monomer, and the S−S bond length is 202.45 pm. The O−S−S angle is 112.7°. S₂O₂ has a dipole moment of 3.17 D.