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I was at a chemistry competition, and one of the questions they asked was on Berry pseudorotation

Now the question gave 4 molecules, namely, $\ce{IF5, PF5, PCl5, and XeF5-}$, and asked which of these molecules is unable to undergo Berry pseudorotation. I immediately eliminated $\ce{PF5 and PCl5}$ as they have trigonal bipyramidal structure, so that left me with the other two molecules.

At this point, I am unable to determine which of the remaining molecules should be the answer, so I take a guess and select $\ce{IF5}$, as my reasoning is that it has lone pairs so it is unable to interconvert. (However this also could be used to explain for why $\ce{XeF5-}$ is a possible answer too)

After reading the Wikipedia article above, it turns out $\ce{IF5}$ is able to undergo such pseudorotation, which leaves me surprised, but in that case, could someone explain why $\ce{XeF5-}$, as opposed to $\ce{IF5}$, is unable to undergo such rotation? (I think it has something to do with the lone pairs on $\ce{XeF5-}$, but I'm not sure)

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    $\begingroup$ It would appear that $\ce{XeF5^-}$ has all 6 atoms co-planar (D$_{5h}$) so would not fit the criterion needed for pseudo-rotation. (See JACS 1991, vol 113, p 3351) $\endgroup$ – porphyrin May 13 '18 at 12:45

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