Seesaw and Triganol Bipyramid

[Macy Matsukawa 3J]

Hello. In lecture today, I did not clearly understand the difference between seesaw shape and triganol bipyramid shape. For example SF₄, is it seesaw shape because of the one lone pair on sulfur? Could I just tell the difference by saying that a triagonal bipyramid shape has 5 regions of electron density with no pairs of lone electrons and seesaw shape also has 5 but one of them is a pair of lone electrons?

[Pauline Tran 2G]

Yes, so the main difference between the seesaw shape and the trigonal bipyramidal shape is the fact tat the seesaw shape has a lone pair in the equatorial position whilst the trigonal bipyramidal shape does not have a lone pair.

[Ronald Yang 2F]

Trigonal bipyramidal and seesaw shapes have the same number of regions of electron density, as you said: 5. If you were to look at PCl₅ and SF₄, both have an electron shape of trigonal bipyramidal, as they both have 5 regions of electron density. However, PCl₅ has no lone pairs, whereas SF₄ has one lone pair. Therefore, when we look at molecular shape, that one lone pair makes SF₄ differ in molecular shape. PCl₅ would still have a molecular shape of trigonal bipyramidal, but SF₄, due to its lone pair, has one of its equatorial atoms replaced with a lone pair, yielding a molecular shape of a seesaw.

[Jennifer Cheng 2L]

The trigonal pyramidal structure actually has four areas of electron density. Take SO₃^2- for example. The sulfur atom is connected to three oxygen atoms (3 areas of electron density) and then it has a lone pair (1 area of electron density). So that gives it a total of four areas of electron density. This gives it the shape of trigonal pyramidal. There are only four atoms involved in this structure: sulfur and three oxygen atoms.

**Edit: Sorry I read trigonal bipyramidal as trigonal pyramidal. Trigonal bipyramidal also has five areas of electron density but all five are bonded pairs.

In contrast, SF₄ has five atoms involved in its structure: a sulfur atom and four oxygens. There are five areas of electron density (compared to four areas of electron density in SO₃^2-). The sulfur atom is bonded to four fluorine atoms (4 areas of electron density) and has one lone pair (1 area of electron density). This gives it the shape of seesaw.

It's helpful to draw out what the structures would look like if there was a bonded atom in place of the lone pair for visualization purposes. Hope this helped.

[Ronald Yang 2F]

The trigonal pyramidal structure actually has four areas of electron density.

You are totally correct, but don't mix up trigonal pyramidal with trigonal bipyramidal.