VSEPR Models with 5 bonding regions

[Annalyn Diaz 1J]

It’s pretty easy to see the trends when it comes to VSEPR models with three or even four bonding regions (lone pairs included) but why does having five bonding regions make these trends harder to see? Or rather, why does having 5 domains somehow break the trend?

[Stephen Sirmay 1I]

When you have bonding pairs including a lone pair, for example four bonding pairs and one lone pair, the correct shape is that with the least amount of repulsion (more stable). This may be why the trend you're talking about is broken. 5 pairs electrons are located in trigonal bipyramidal geometry. Lone pair in equatorial plane (more repulsive with only 2 bonds at 90 degrees). Since only 4 of 5 positions are occupied by atoms, these 4 atoms form the shape of a seesaw.

[michelle]

I think it became complicated from 5 bonding regions because the increasing chance of lone pair getting involved. You can simply first ignore the difference between bonded and lone pair regions and regard them all as atoms to get the intuitive shape and remove the atom to reconsider if it is a lone pair following the rule of VSEPR.

[Cade Okohira 4K]

When there are 5 bonding regions, the model can be either AX5, AX4E, AX3E2, or AX2E3. For AX5, its molecular geometry is trigonal bipyramidal, with 3 atoms in a plane in equatorial positions and two atoms above and below the plane in axial positions. For AX4E, the lone pair of electrons in the equatorial position is more stable than the one with the lone pair in the axial position, so the geometry would be the seesaw. For Ax3E2, the geometry is essentially a trigonal bipyramidal without its 2 equatorial vertices because of the 2 lone pairs.