https://doi.org/10.1351/goldbook.A00415
In trigonal bipyramidal structures (e.g. a five-coordinate trigonal bipyramid with phosphorus as ![Not defined](/img/inline/A00415.png)
central atom
) the term apical refers to the two positions that are collinear with the central atom
or to the bonds linking these positions to the central atom
. The three equivalent bonds (or positions) in a plane passing through the central atom
and perpendicular to the direction of the apical bonds are described as equatorial
. (See axial, equatorial
for alternative use.). The term apical is also used for the bond pointing from the atom at or near the centre of the base to the apex
of a pyramidal structure. The positions at or near the base of the pyramid, or the bonds linking those positions to the central atom
of the base are described as basal
. The apical bonds have also been called axial. ![Not defined](/img/inline/A00415.png)