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I've been learning about using MO theory to explain why $\ce{BH3}$ and $\ce{NH3}$ have different geometries and by following the line of reasoning used to rationalise the differences in geometry I came to wonder whether ionisation or the addition of electrons to a molecule can change its geometry.

Eg: $\ce{NH3}$ is trigonal pyramidal but an $\ce{NH3^2+}$ ion would have the same electron configuration as $\ce{BH3}$, which is trigonal planar; does this mean $\ce{NH3^2+}$ would be trigonal planar?

Edit: My bad when I first posted this I wrote $\ce{NH3^2-}$ but meant $\ce{NH3^2+}$, this was just an example, my main objective was to find out whether changing the geometry through ionisation was possible.

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    $\begingroup$ The short answer is: yes. The slightly longer answer is: a molecule always reacts to a change within itself or the environment. $\endgroup$
    – Martin - マーチン ♦
    Aug 2, 2018 at 18:51
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    $\begingroup$ It would likely launch off proton and become H2N+ $\endgroup$
    – permeakra
    Aug 2, 2018 at 20:42

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Not only is changing shape due to ionization possible, it has been observed. Not with ammonia as suggested in the question, but with cyclooctatetraene, which is nonplanar in its neutral state but switches to a planar, aromatic ring structure when reduced to its di-anion.

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