Using the obsolete VSEPR theory I had a hard time predicting the structure of $\ce{H2XeO4}$. So I looked it up. I found the molecule via chemapps.stolaf.edu:


It looks weird to me. I can't explain the molecular structure. Can anyone explain it using more sophisticated theories like molecular orbital theory (MOT)?
I don't know much MOT, yet.

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    $\begingroup$ Interesting, do you know where the structural data (that chemapps.stolaf.edu uses) comes from? Atkins' Chemical principles: the quest for insight, p. 661 (ISBN 978-1-4292-1955-6) depicts xenic acid as tetrahedron, and to the best of my knowledge there is still no single crystal diffraction data available elsewhere. $\endgroup$
    – andselisk
    May 18, 2019 at 6:32
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    $\begingroup$ I knew that oxo-acids of Xe are very unstable and don't really exist. This is the reason that upon hydrolysis of XeFₓ we get oxides of Xe as products and not oxo-acids. $\endgroup$
    – user73099
    May 18, 2019 at 7:32
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    $\begingroup$ en.wikipedia.org/wiki/Xenic_acid $\endgroup$ May 18, 2019 at 7:45
  • $\begingroup$ @andselisk Even chemtube 3d shows the same. It computed (predicted) the structure I guess. chemtube3d.com/playground/playground.html $\endgroup$ May 18, 2019 at 7:52
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    $\begingroup$ Please note that the VSEPR theory is not obsolete. It has it's limitations, and this is one of them, but if you are applying it within reason, then it is a very useful model. $\endgroup$ May 29, 2019 at 12:50

1 Answer 1


When I think about such structure, I draw all valence electrons around the atoms and connect them somehow reasonably. There are two ways to connect the hydrogens to xenon. Either directly bonded as H-Xe bonds, which for me does not feel right here, or as part of OH groups, which also make more sense in the "acid" part. By connecting everything that way, you end up with:

crappy hand drawn impression of H2XeO4

This let you end up with a lone pair at xenon which is the reason for the non-tetrahedral structure.

A quick and dirty quantum calculation[1] shows what I mean. By calculating the ELF (Electron Localization Function), you can easily see where the electrons are located. The lone pair is shown by the yellow volume surrounding no atom. b3lyp def2-QZVP//xtb 6.2

This structure is not clear from the Lewis formula shown on Wikipedia, but can be seen in the ball and stick representation that is shown there (don't care about the "flipped" hydrogens):

xenic acid by wikipedia

[1] ORCA 4.1.2, best functional of the planet B3LYP def2-QZVP // Crest 2.6, Visualization with Chimera, iso value 0.8 ... whatever ... camels? ... what unit has the ELF?

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    $\begingroup$ It's a probability distribution, so the unit is 1 (or none), and values are 0 to 1. (And B3LYP.... pffft) $\endgroup$ May 27, 2019 at 21:58

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