aBetterChemText: VSEPR Applied (5 things)

When 5 things are found around the central atom, as in the molecule PF5, the structure is called a trigonal bipyramid This can be seen as a trigonal planar structure with one atom above the plane of the planar triangle and one below. (The lone pairs on the fluorines have been left off so that the structure is as clear as possible.

The shape of this molecule is, of course, also a trigonal bi pyramid This is the only structure in which all of the angles are not identical.

The three angles around the middle are all 120^o, as in a trigonal planar structure, while the atoms above and below the plane are 90^o to each of the atoms in the plane.

When a molecule contains 5 things, one of which is a lone pair, as in the molecule SF4, the structure is still a trigonal bi pyramid, but determining the shape requires more thought. For the first time, there is a choice as to where the lone pair goes.

Remembering that a lone pair of electrons takes up more space than a bonded pair, we can compare the two possible placements of the lone pair. If the lone pair takes one of the spots either above or below the plane, it will have 3 neighbors at 90^o. If, on the other hand, it is placed in the plane, it will have 2 neighbors at 120^o and 2 neighbors at 90^o. Neighbors at 90^o are so much closer than those at 120^o, that there is a clear advantage (less repulsion) to having 2 close neighbors rather than 3. For this reason, the lone pair is always found IN the plane rather than above or below it. This give the shape seen below.

If we turn this figure on its side, we can see why this shape is called see-saw. (I've added handles in the second drawing just to make the shape that much more obvious.)

When a molecule contains 5 things, two of which are lone pairs, as in IF3, the structure is, of course, still trigonal bipyramid. As before, the lone pairs will both go into the plane, producing a T-shaped molecule.