Sulfur monoxide is an inorganic compound with formula SO. It is only found as a dilute gas phase. When concentrated or condensed, it converts to S2O2 (disulfur dioxide). It has been detected in space but is rarely encountered intact otherwise.
|Systematic IUPAC name|
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||48.064 g mol−1|
|221.94 J K−1 mol−1|
Std enthalpy of
|5.01 kJ mol−1|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Structure and bonding
The SO molecule has a triplet ground state similar to O2, i.e. each molecule has two unpaired electrons. The S−O bond length of 148.1 pm is similar to that found in lower sulfur oxides (e.g. S8O, S−O = 148 pm) but is longer than the S−O bond in gaseous S2O (146 pm), SO2 (143.1 pm) and SO3 (142 pm).
The molecule is excited with near infrared radiation to the singlet state (with no unpaired electrons). The singlet state is believed to be more reactive than the ground state triplet state, in the same way that singlet oxygen is more reactive than the triplet oxygen.
Production and reactions
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: as well as more complex examples, such as a trisulfide oxide, C10H6S3O,
- C2H4S=O → C2H4 + S=O
Generation under extreme conditions
In the laboratory sulfur monoxide can be produced by treating sulfur dioxide with sulfur vapour in a glow discharge. It has been detected in single bubble sonoluminescence of concentrated sulfuric acid containing some dissolved noble gas.
A chemiluminescence detector for sulfur has been reported that is based on the reactions:
- SO + O3 → SO2(excited) + O2
- SO2(excited) → SO2 + hν
Ligand for transition metals
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 the Hale-Bopp comet and in the interstellar medium.
- O + S2 → S + SO
- SO2 → SO + O
Sulfur monoxide has been found in the largest star known, NML Cygni.
Because of sulfur monoxide's rare occurrence in our atmosphere and poor stability; it is difficult to fully determine its hazards. But when condensed and compacted, it forms disulfur dioxide, which is relatively toxic and corrosive. This compound is also highly flammable (similar flammability to methane) and when burned produces sulfur dioxide, a poisonous gas.
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 essientially move barrierless over the benzene ring. The S-O bond length is 1.424(2) Å.
SO converts to disulfur dioxide (S2O2). Disulfur dioxide is planar molecule with C2v 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 OSS angle is 112.7°. S2O2 has a dipole moment of 3.17 D.
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