Why does the $\ce{XeF6}$ molecule have an octahedral structure while $\ce{XeOF5-}$ has a pentagonal pyramidal structure, despite both molecules having 7 electron pairs? I suppose the extra electron on oxygen would distort the molecule somewhat, but this seems like an extreme outcome.


$\ce{XeF6}$ is not octahedral, but distorted (or capped octahedral), one of the possible arrangement for seven electron pairs.

Wikipedia provides the following, quoted from a paper by Konrad Seppelt in Accounts of Chemical Research[1]:

...the structure is best described in terms of a mobile electron pair that moves over the faces and edges of the octahedron and thus distorts it in a dynamic manner...

The molecular geometry of $\ce{XeF6}$ is thus not truly an octahedron, nor is it static. In fact, the referenced article by Seppelt indicates that $\ce{XeF6}$ is only monomeric in the gas phase. In solution and the solid phase it is tetrameric $\ce{Xe4F24}$ clusters. The solid phase appears to be more like clusters of $\ce{[XeF5+ F^{-}]}$ with the fluoride ions bridging two $\ce{XeF5+}$ centers. Thus, in the solid, every $\ce{Xe}$ atom has 7 bonds to fluorine, with two that are more ionic. The structure for this scenario looks like the pentagonal pyramid.


  1. Seppelt, Konrad (June 1979). "Recent Developments in the Chemistry of Some Electronegative Elements". Accounts of Chemical Research 12 (6): 211–216. doi:10.1021/ar50138a004
| improve this answer | |
  • $\begingroup$ In XeF6 monomeric structure where will the lone pairs be situated ? In the equatorial plane or axial plane...? $\endgroup$ – Subhadip Pal Aug 23 '15 at 4:25
  • 2
    $\begingroup$ @SubhadipPal - see the block quote in my answer. The lone pair is mobile (or perhaps delocalized). $\endgroup$ – Ben Norris Aug 23 '15 at 10:17
  • $\begingroup$ Can u explain the structure by VSEPR thoery..? $\endgroup$ – Subhadip Pal Sep 9 '15 at 17:53
  • 2
    $\begingroup$ @SubhadipPal - VSEPR is a model based on classical mechanics that survives into the quantum mechanics age. VSEPR survives because it is a useful model, but it has its limits. This compound is one of its limits. $\endgroup$ – Ben Norris Sep 9 '15 at 18:02

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