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.

  • 1
    $\begingroup$ 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 '19 at 6:32
  • 1
    $\begingroup$ 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 '19 at 7:32
  • 1
    $\begingroup$ en.wikipedia.org/wiki/Xenic_acid $\endgroup$ – Nilay Ghosh May 18 '19 at 7:45
  • $\begingroup$ @andselisk Even chemtube 3d shows the same. It computed (predicted) the structure I guess. chemtube3d.com/playground/playground.html $\endgroup$ – Karthik May 18 '19 at 7:52
  • 2
    $\begingroup$ 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. $\endgroup$ – Martin - マーチン May 29 '19 at 12:50

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?

| improve this answer | |
  • 1
    $\begingroup$ It's a probability distribution, so the unit is 1 (or none), and values are 0 to 1. (And B3LYP.... pffft) $\endgroup$ – Martin - マーチン May 27 '19 at 21:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.