Disulfur dioxide, dimeric sulfur monoxide or SO dimer is an

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

with the formula S₂O₂. The solid is unstable with a lifetime of a few seconds at room temperature.

Structure

Disulfur dioxide adopts a ''cis'' planar structure with ''C''_2v

symmetry Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is Invariant (mathematics), invariant und ...

. The S−O bond length is 145.8 pm, shorter than in sulfur monoxide. The S−S bond length is 202.45 pm and the O−S−S angle is 112.7°. S₂O₂ has a dipole moment of 3.17 D. It is an asymmetric top molecule. The electronic ground state is a singlet, unlike disulfur or

dioxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are: * Ato ...

Formation

Sulfur monoxide (SO) converts to disulfur dioxide (S₂O₂) spontaneously and reversibly. So the substance can be generated by methods that produce sulfur monoxide. Disulfur dioxide has also been formed by an electric discharge in

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

. Another laboratory procedure is to react oxygen atoms with

carbonyl sulfide Carbonyl sulfide is the chemical compound with the linear formula . It is a colorless flammable gas with an unpleasant odor. It is a linear molecule consisting of a carbonyl double bonded to a sulfur atom. Carbonyl sulfide can be considered to ...

carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is an inorganic compound with the chemical formula and structure . It is also considered as the anhydride of thiocarbonic acid. It is a colorless, flammable, neurotoxic liquid that is used as ...

vapour. Although most forms of elemental sulfur ( S₈ and other rings and chains) do not combine with SO₂, atomic sulfur does so to form sulfur monoxide, which dimerizes: :S + SO₂ → S₂O₂ ⇌ 2 SO Disulfur dioxide is also produced upon a microwave discharge in sulfur dioxide diluted in

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

. At a pressure of , five percent of the result is S₂O₂. Disulfur dioxide is formed transiently when

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

and oxygen undergo flash photolysis. A branched isomer isoelectronic to SO₃, S=SO₂, is believed to form during the thermal decomposition of cyclic vicinal alkyl thiosulfites.

Properties

The

ionization energy In physics and chemistry, ionization energy (IE) is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, Ion, positive ion, or molecule. The first ionization energy is quantitatively expressed as : ...

of disulfur dioxide is . Disulfur dioxide absorbs at 320–400 nm, as observed of the Venusian atmosphere, and is believed to have contributed to the

greenhouse effect The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source (as in the case of Jupiter) or ...

on that planet.

Reactions and decomposition

Although disulfur dioxide exists in equilibrium with sulfur monoxide, it also reacts with sulfur monoxide to form

and disulfur monoxide. Decomposition of S₂O₂ proceeds via the following disproportionation reaction: :

Complexes

S₂O₂ can be a

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

with transition metals. It binds in the ''η''²-S–S position with both sulfur atoms linked to the metal atom. This was first shown in 2003. The bis(trimethylphosphine) thiirane ''S''-oxide complex of

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

, when heated in

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...

at 110 °C loses

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

, and forms a complex with S₂O₂: (Ph₃P)₂Pt(S₂O₂).

Iridium Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group, is considered the second-densest naturally occurring metal (after osmium) with a density ...

atoms can also form a complex: ''cis''- dppe)₂IrS₂l with sodium periodate oxidizes to dppe)₂IrS₂Oand then to dppe)₂IrS₂O₂ with dppe being 1,2-bis(diphenylphosphino)ethane. This substance has the S₂O₂ in a ''cis'' position. The same conditions can make a ''trans'' complex, but this contains two separate SO radicals instead. The iridium complex can be decomposed with triphenylphosphine to form triphenylphosphine oxide and triphenylphosphine sulfide.

Anion

The radical

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

has been observed in the gas phase. It may adopt a

trigonal In crystallography, the hexagonal crystal family is one of the six crystal family, crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the tr ...

shape akin to SO₃.

Spectrum

Microwave

In the Solar System

There is some evidence that disulfur dioxide may be a small component in the

atmosphere of Venus The atmosphere of Venus is the very dense layer of gases surrounding the planet Venus. Venus's atmosphere is composed of 96.5% carbon dioxide and 3.5% nitrogen, with other chemical compounds present only in trace amounts. It is much denser and ho ...

, and that it may substantially contribute of the planet's severe

. It is not found in any substantive quantity in Earth's atmosphere.

References

{{Oxides Sulfur oxides Sulfur(II) compounds