Trigonal Bipyramidal with 1 lone pair and 4 bonds

[Hannah Guo 3D]

Prof. Lavelle talks about when there is one lone pair of electrons and 4 bonds in a trigonal bipyramidal molecule (AX4E), the lone pair is more likely to be in the equatorial plane because the lone pair in this way has less repulsion and is more stable. Can anyone explain it in detail that why the lone pair is less repulsive in the equatorial plane? Thank you!

[Nishma Chakraborty 1J]

Hi!

So, lone pair - bond pair repulsions are by nature stronger than bond pair - bond pair repulsions. This is because lone pairs of electrons are closer to the positively-charged nucleus, so they repel each other more. In order for the atom to stay as stable as possible, it places the more repulsive lone pairs in the state where they have more “space” (equatorial plane).

Hope this helps! :)

[Rachel Formaker 1E]

In the equatorial plane, there are 3 regions of electron density, which can be separated by approximately 120^o bond angles.

This is favorable compared to axial atoms above and below the equatorial plane, which have only approximately 90^o bond angles with equatorial atoms.

Since lone pairs have more repulsion, it is most stable for them to be in the location where they are farthest from the other atoms/regions of electron density. This is in the equatorial plane in a molecule with a trigonal bipyramidal shape.

[Ruohan Sun 3I]

Because on the equatorial plane the lone pair only has 2 bonds which are 90° from it, but if it is on the axis it will have 3. So the lone pair repels less bonds on the equatorial plane.