Sulfur monoxide

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 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 thiirane 1-oxide: as well as more complex examples, such as a trisulfide oxide, C₁₀H₆S₃O,

C₂H₄OS → C₂H₄ + SO

The SO molecule is thermodynamically unstable, converting initially to S₂O₂. SO inserts into alkenes, alkynes and dienes producing molecules with three membered rings containing sulfur.

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

As a ligand SO can bond in a number different ways:

a terminal ligand, with a bent M-S-O arrangement, analogous to bent nitrosyl

bridging across 2 or 3 metal centres (via sulfur), as in Fe₃S(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 the Hale-Bopp comet 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 the largest star known, NML Cygni.

Biological chemistry

Sulfur monoxide may have some biological activity, the formation of transient SO in porcine coronary artery has been inferred from the reaction products.

Safety measures

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 sulphur dioxide, a poisonous gas.

Disulfur dioxide

SO converts to disulfur dioxide (S₂O₂). Disulfur dioxide is 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 OSS angle is 112.7°. S₂O₂ has a dipole moment of 3.17 D.