CHEM 121 glossary

Trigonal bipyramidal

Five domains: three equatorial positions in a plane at 120°, plus two axial positions above and below, at 90° to the plane and 180° to each other. It is the electron-domain geometry for SN = 5 and the shape of $\mathrm{AX_5}$ .

\ce{PCl5}

: 120° in the equator, 90° between axial and equatorial.

Example

$\ce{PF5}$ and $\ce{AsF5}$ are trigonal bipyramidal. With 1, 2 or 3 equatorial lone pairs the shape becomes seesaw ( $\ce{SF4}$ ), T-shaped ( $\ce{ClF3}$ ) or linear ( $\ce{XeF2}$ ).