Weakness of S−I and Se−I bonds

After recent question about non-existent pnictide(V) iodides, I stumbled upon a similar issue in oxygen group. For example, $\ce{SI2}$ was reported to be found… but at the temperature as low as $\pu{9 K}.$ Well, OK, I heard that sulfur prefers disulfides, but [$\ce{S2I2}$](https://en.wikipedia.org/wiki/Disulfur_diiodide) was characterised at $\pu{-90 °C}.$ Moreover, situation with selenium iodides is even worse!


One of the reasons for that is weakness of $\ce{S-I}$ and $\ce{Se-I}$ bonds: their respective enthalpies are $\pu{-18 kJ/mol}$ and $\pu{-1.3 kJ/mol}.$ Well, that looks more like values for *hydrogen* bonds…


Klapötke and Passmore [[1, p. 234](https://doi.org/10.1021/ar00163a002)] provide an explanation for the weakness of these bonds:



> 
> The instability of the $\ce{S-I}$ and $\ce{Se-I}$ bonds can be attributed to their very low ionic resonance stabilization energies
> as the electronegativity of iodine is about the same as that of sulfur and selenium.
> 
> 
> 


I think this is a partial explanation, at best - ionic component sure makes bonds stronger, but by this logic all non-polar covalent bonds should be weak. I’m not saying I’d expect bonds created by iodine to be strong, but it’s not like we need cryogenic temperatures to get methyl iodide!


Could anyone provide a *good* explanation as to why these bonds are so weak, maybe describing stereo-electronic effects, or using computational methods?


### References


1. T. Klapötke, J. Passmore, J. *Acc Chem. Res.* **1989**, *22*, 234–240. [DOI:10.1021/AR00163A002](https://doi.org/10.1021/ar00163a002).




It isn't just sulfur iodide. Sulfur in low oxidation states does not form any $\ce{SX2}$ molecules that are stable under ambient or near-ambient conditions. Such molecules tend to dimerize, with loss of some halogen atoms in the case of chlorine or a heavier halogens, forming disulfur compounds. (In the case of $\ce{S2I2}$, [Wikipedia](https://en.wikipedia.org/wiki/Disulfur_diiodide) reports stability up to a somewhat higher temperature than suggested in the question,
$\pu{-30 °C}).$


What happens is sulfur in the disulfur compounds can act as a pi donor, thereby strengthening the sulfur–sulfur bond while only slightly weakening the sulfur–halogen bond. The specific case of disulfur dichloride is discussed [here](https://chemistry.stackexchange.com/a/165021/17175).


In addition to the halides, the structure of [disulfur monoxide](https://en.wikipedia.org/wiki/Disulfur_monoxide), which appears to exist in the Jovian moon Io, allows a similar interaction between the sulfur–sulfur bond and oxygen.


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