Seesaw Bond Angles

[Dan Dinh 3K]

I know that for a seesaw shape, it originates from the trigonal bipyramidal shape. Where do you put the lone pair and why? Also, does this affect the bond angles for seesaw, because my TA said it would still be 90 degrees and 120 degrees?

[Divya Prajapati 1E]

For the seesaw shape, we have 5 regions of electron density (trigonal bipyramidal), consisting of 4 bonding pairs and 1 lone pair. Normally, when all these regions are bonding, the molecule has 120 degree angles between the three atoms making up the "trigonal" part of the shape and 90 degree angles between the two atoms of the "bipyramidal" part of the shape in relation to the other atoms.

When a lone pair is added, we want to place it as far away as possible from the bonding pairs due to electron-electron repulsion. When we place it in the "trigonal" part of the shape, there is an approximately 120 degree angle between the lone pair and other two bonding pairs in the same plane, and a 90 degree angle with the other two perpendicular bonding pairs. In contrast, if we placed the lone pair at the "top" or "bottom" (perpendicular plane) of the molecule, every bond angle would be 90 degrees, which would be much smaller than some of the 120 degree angles possible with the other placement.

So, that's why we put the lone pair in the same plane as the two atoms comprising the "trigonal" portion of the atom.

As to the altered degrees of each bond, the bond angles should be a bit smaller due to electron-electron repulsion. Just knowing they are less than 90 degrees and 120 degrees should be enough.

[Angela Cong 3C]

would we also need to include the 180 degree angle when they ask for the bond angles?

[Jonathan Pai 2I]

Nah

[Sam Kelly 1K]

Probably wouldn't hurt to list 180 as a bond angle

[Ray Guo 4C]

the shape should look like this

[MichelleRamirez_2F]

The lone pair is placed on the axis of the molecule. This is the Axail lone pair and it is placed in this position because of the repulsion of the atoms from the electron pair.