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I read that group-16 monohalides exist in dimeric forms ex. $\ce{S2Cl2,Se2Cl2,Se2Br2}$ etc.
It also states that these dimeric halides undergo disproportionation reactions and gives an example as follows: $\ce{2Se2Cl2 -> SeCl4 + 3Se}$

Now I thought what would be the disproportion reaction if we consider the case of $\ce{S2Cl2}$ Since $\ce{S(VI)}$ is more stable than $\ce{S(IV)}$ would the disproportion reaction in case of $\ce{S2Cl2}$ yield $\ce{SCl4}$ or $\ce{SCl6}$?

EDIT: As pointed out in the comments. $\ce{SCl6}$ doesnt exist hence the same question is now considering the case of decomposition of $\ce{S2F2}$ with the essence of question still remaining the same

Ashish
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    (1) Here is a different pathway for diselenium dichloride disproportionation which yields $\ce{SeX2}$ and $\ce{Se_{n}X2}$ (2) In this patent, it is mentioned "at higher temperatures, sulfur monochloride tends to disproportionate to form sulfur and sulfur dichloride" (3) SCl6 doesn't exist. – Nilay Ghosh May 07 '21 at 06:26
  • Didn't consider that $\ce{SCl6}$ doesn't exist but what if we talk about $\ce{S2F2}$ ? – Ashish May 07 '21 at 06:47
  • With S2F2 you'll have a similar reaction. – Ivan Neretin May 07 '21 at 08:40
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    @Ashish Wikipedia says that it decompose to sulfur tetrafluoride ($\ce{S^{(IV)}F4}$) and elemental sulfur when heated. Also, a 1974 paper (I don't have access) suggested that it disproportionate to sulfur difluoride ($\ce{S^{(II)}F2}$) and sulfur. – Nilay Ghosh May 07 '21 at 09:27
  • But isn't VI more stable O.S for sulphur ? And considering the fact that we say that "SF6 is exceptionally stable" shouldn't that form ? Or is it that SF6 is just relatively more stable than other hexafluorides (halides) and not more than SF4 ? Or does it not depend on stability at all ? – Ashish May 07 '21 at 12:08
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    O.S. is a made-up concept made by humans. It does not account for stability of a particular compound. SF6 is stable not because it has sulfur in its +6 O.S. but it is due to steric reasons. Sulfur is surrounded by six fluorine atoms so it has no space for sulfur to get attacked, hence you do not see it getting hydrolyzed. This can also explain inexistence of SCl6 because you can't fit six chlorine atoms to sulfur. See Ivan's answer to get an idea. – Nilay Ghosh May 07 '21 at 12:18

1 Answers1

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Compiling my comments into an answer, sulfur monohalide disproportionate to sulfur dihalide and/or sulfur tetrahalide and elemental sulfur.

Wikipedia has the reaction:

$$\ce{2S2F2 ->[180 ^\circ C] SF4 + 3S}$$

Also, there is a reaction scheme from a 1974 entry1 which indicates formation of octasulfur and sulfur difluoride as sideproduct.

$$\ce{12S2F2 -> 3S8 + 8SF4}$$ $$\ce{8S2F2 -> S8 + 8SF2}$$

It also further noted that this disproportionation reaction is useful in the synthesis sulfur tetrafluoride and the decomposition of two moles of $\ce{S2F2}$ into one mole of $\ce{SF4}$ and sulfur yields $\pu{\Delta H^\circ = -24.6 kcal/mole}$.

Furthermore, sulfur difluoride may disproportionate to sulfur tetrafluoride and sulfur or some other oligomer/polymer of sulfur or back to sulfur monofluoride or isomerize to various sulfur compounds like $\ce{FSSF}$, $\ce{SSF2}$ or $\ce{SF3SF}$. These compounds are extremely unstable and prone to hydrolysis:

$$\ce{16SF2 -> 8SF4 + 8S8}$$ $$\ce{3SF2 -> SF4 + S2F2 isomers}$$

The stability of $\ce{SF2}$ can be considered thermodynamically. The decomposition of two moles of $\ce{SF2}$ to yield $\ce{SF4}$ and $\ce{S8}$(gas) is $\pu{-75.7 kcal/mole}$. Another reason for this decomposition is polymerization which forms new sulfur-fluorine bonds. Also, it is noted that $\ce{SF4}$ is thermodynamically unstable and disproportionate to sulfur hexafluoride and sulfur:

$$\ce{24SF4 -> 16SF6 + S8~~~~\Delta H^\circ = -62.9 kcal/mole}$$

So, sulfur compounds in these intermediate oxidation state are found to be thermodynamically unstable and it is also influenced by its kinetic stability.

Sulfur monochloride disproportionation occurs in a similar way as noted in this patent:

Elevated temperature as used herein refers to temperatures between about 75 and about 150C; and preferably between about 85 and about 138C. Higher temperatures can be used but the sulfur monochloride will tend to disproportionate to form sulfur and sulfur dichloride. lf higher amounts of sulfur in the sulfur monochloride product are not objectionable, higher temperatures can be utilized

Notes and References

  1. Advances in Inorganic Chemistry and Radiochemistry, Academic Press, 1974
  2. There is a different pathway for disproportionation of selenium monochloride/bromide where it is seen to yield $\ce{SeX2}$ and $\ce{Se_{n}X2 (n =3,4)}$ unlike the general pathway you mentioned which yields $\ce{SeX4}$ and elemental selenium. See this paper for more details.
Nilay Ghosh
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