The pentagonal pyramidal structure of XeOF5- [closed]








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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.




$\ce{XeF6}$ is not octahedral, but distorted (or capped octahedral), one of the possible arrangement for [seven electron pairs](https://winter.group.shef.ac.uk/vsepr/high-cn.html).


[Wikipedia](https://en.wikipedia.org/wiki/Xenon_hexafluoride) provides the following, quoted from a paper by Konrad Seppelt in Accounts of Chemical Research[1]:



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> ...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...
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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:



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> 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](https://dx.doi.org/10.1021%2Far50138a004)
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