Sulfur monoxide

    Sulfur monoxide is a chemical compound with formula SO and CAS number 13827-32-2. It is an unstable species only found in the gas phase where it is in equilibrium with a dimeric form, S₂O₂ (sometimes called disulfur dioxide) which can be represented as OSSO^[1]. Sulfur monoxide has been detected around Io, one of Jupiters moons, both in the atmosphere^[2] and in the plasma torus^[3]. It has also been found in the atmosphere of Venus^[4], in the Hale-Bopp comet^[5] and in interstellar space^[6].

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Structure and bonding

The SO molecule has a triplet ground state similar to dioxygen, O₂, with two unpaired electrons^[7]. The S−O bond length of 148.1pm is similar to that found in lower sulfur oxides (e.g. S₈O, S−O =148pm) but is longer than the S−O bond in gaseous S₂O (146pm), SO₂ (143.1pm) and SO₃(142pm)^[7].
The S₂O₂ dimer is planar molecule with C_2v symmetry, with two SO units joined via the sulfur atoms, and the oxygen atoms in a cis configuration. The S-O bond length is 145.8pm, shorter than in the monomer, and the S-S bond length is 202.45pm. The OSS angle is 112.7 degrees. It has a dipole moment, μ = 3.17D^[1].

Production and reactions

In the laboratory sulfur monoxide be made reacting sulfur dioxide with sulfur vapour in a glow discharge ^[7] and in single bubble sonoluminescence of concentrated sulfuric acid containing some dissolved noble gas^[8].
Production of the transient SO molecule as a reagent in organic syntheses has been investigated. Research has centred on using compounds that decompose under reaction conditions to "extrude" SO. Examples include the decomposition of the relatively simple molecule thiirane 1-oxide^[9]:

C₂H₄OS → C₂H₄(g) + SO(g)

as well as more complex examples e.g. a trisulfide oxide, C₁₀H₆S₃O, ^[10]
On Io the production of SO is believed to be both volcanic and also photochemical. The principal photochemical reactions proposed ^[11] are:

O + S₂ → S + SO

SO₂ + hν → SO +O

The SO molecule is thermodynamically unstable ^[7]. The transient SO molecule can be trapped by the formation of transition metal complexes. As a ligand SO can bond in a number different ways^[12]:

• σ donation forming a bent bond with the metal
• bridging across 2 or 3 metal centres via sulfur
• edge on bridging

SO inserts into alkenes, alkynes and dienes producing molecules with three membered rings containing sulfur.^[13]
The triplet ground state molecule with two unpaired electrons can be excited by near infrared light 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 ground state^[14].
Sulfur monoxide may have some biological activity, the formation of transient SO in porcine coronary artery has been inferred from the reaction products^[15].

Uses

A chemiluminescence detector for sulfur has been reported^[16]that is based on the reactions:

SO + O₃ → SO₂(excited) + O₂

SO₂(excited) → SO₂ + hν

References