# What is the explanation of the structure of H2XeO4?

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.

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

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.

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):

[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?

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