# Disulfur monoxide

Disulfur monoxide or sulfur suboxide is an inorganic compound with the formula S$_2$O, one of the lower sulfur oxides. It is a colourless gas and condenses to give a roughly dark red coloured solid that is unstable at room temperature.
S $_2$O occurs rarely in natural atmospheres, but can be made by a variety of laboratory procedures. For this reason, its spectroscopic signature is very well understood.

## Structure and spectrum

Like sulfur dioxide (and, indeed, most molecules) but unlike sulfur monoxide, disulfur, or dioxygen, the ground state of disulfur monoxide is a singlet.
Condensed solid S$_2$O absorbs at 420 nm (roughly indigo) and 530 nm (roughly lime). These bands have been assigned to decomposition products S$_3$ and S$_4$.
In the ultraviolet, S$_2$O has absorption band systems in the ranges 250–340 nm and 190–240 nm. There are bands at 323.5 and 327.8 nm. The band in the 315–340 nm range is due to the C$^1$A′–X$^1$A′ (π* ← π) transition.
Gaseous disulfur monoxide does not absorb light in the visible spectrum.
The microwave spectrum of S$_2$O has the following rotational parameters: A = 41915.44 MHz, B = 5059.07 MHz, and C = 4507.19 MHz. Moreover, the microwave spectrum suggests the S−S−O angle is 117.88° with S−S and S−O bond lengths of 188.4 and 146.5 pm, respectively. In the 327.8 nm excited state, the central angle tightens to 109°.
The harmonic frequency for S−S stretching is 415.2 cm$^{−1}$.

## Synthesis
### Historical

Disulfur monoxide was discovered by Peter W. Schenk in 1933 with a glow discharge though sulfur vapour and sulfur dioxide. He discovered that the gas could survive for hours at single digit pressures of mercury in clean glass, but it decomposed near 30 mmHg (4 kPa). Schenk assigned the formula as SO and called it sulfur monoxide. In 1956, D. J. Meschi and R. J. Myers established the formula as S$_2$O.

### Preparation

Oxidizing sulfur with copper(II) oxide:
    3 S$_8$ + 12 CuO → 12 CuS + 4 S$_2$O + 4 SO$_2$
A relatively pure generator is the reaction of thionyl chloride with silver(I) sulfide:
    SOCl$_2$ + Ag$_2$S → 2 AgCl + S$_2$O
Also 5,6-di-tert-butyl-2,3,7-trithiabicyclo[2.2.1]hept-5-ene 2-endo-7-endo-dioxide decomposes upon heating with release of S$_2$O:
Triphenylphosphine sulfide reacts with sulfinyltosylimide to give S$_2$O and tosyltriphenylphosphinylamide:
    TsNSO + SPPh$_3$ → TsNPPh$_3$ + S$_2$O

## Occurrence
### Volcanism

Volcanoes on Io produce substantial quantities of S $_2$O. It can form between 1% and 6% when hot 100-bar S$_2$ and SO$_2$ gas erupts from volcanoes. It is believed that Pele on Io is surrounded by solid S$_2$O.

### Terran atmosphere

Disulfur monoxide is too unstable to survive at standard conditions, but transient sources include incomplete combustion of sulfur vapor and thermal decomposition of sulfur dioxide in a glow discharge.

### As a ligand

Disulfur monoxide occurs as a ligand bound to transition metals, typically with hapticity 2. Examples include OsCl(NO)(PPh$_3$)$_2$(S$_2$O); [Ir(PPh$_2$)$_2$(S$_2$O)]$^+$; and MeCpMn(CO$_2$)(S$_2$O). These complexes are closely related to transition metal sulfur dioxide complexes.

## Reactions

On decomposition at room temperature it forms SO$_2$ via the formation of polysulfur oxides:
    2 S$_2$O → "S$_3$" + SO$_2$
S $_2$O reacts with diazoalkanes to form dithiirane 1-oxides.