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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.

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$\ce{XeF6}$ 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.

Reference:

  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
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  • $\begingroup$ 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
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    $\begingroup$ @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
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    $\begingroup$ @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

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