Trigonal bipyramidal
Molecules with elements that can access expanded valence can have 5 VSEPR pairs, resulting in a VSEPR shape of trigonal bipyramidal.
The trigonal bipyramidal structure has two distinct environments: the three VSEPR pairs in the equatorial plane are separated by 120◦, and the two VSEPR pairs in the axial positions, above and below the trigonal plane, are separated by 180◦ or 90◦ between equatorial (green) and axial (purple ) positions. PF5 is an example of a molecule with this structure. It has 5 bond pairs and therefore a VSEPR formula of AX5.
Lone pairs are slightly larger than bonded pairs, and always occupy the positions providing more space. For example, the ideal angles between the positions in the equatorial plane of trigonal bipyramids are larger than the angles of the axial positions. Therefore, lone pairs occupy the more spacious equatorial positions. The space filled model for PF5 (trigonal bipyramidal VSEPR and molecular shape) shows the extra space between the atoms in the equatorial positions.
An example of a molecule with a VSEPR formula of AX4E is BrCl4+, which adopts a trigonal bipyramidal VSEPR shape in which one of the equatorial positions is occupied by a lone pair, because the equatorial positions are more spacious than the axial positions. The resulting molecular shape is called disphenoidal, or see-saw. The two lone pairs in AX3E2 systems, such as ClF3, also occupy equatorial positions for space reasons. Their molecular shape is called T-shaped. The three lone pairs in AX2E3 systems, such as XeF2, all occupy equatorial positions, producing a linear molecular shape with the X atoms in axial positions.