apical (basal, equatorial)

In trigonal bipyramidal structures (e.g. a five-coordinate trigonal bipyramid with phosphorus as @C00930@) the term apical refers to the two positions that are collinear with the @C00930@ or to the bonds linking these positions to the @C00930@. The three equivalent bonds (or positions) in a plane passing through the @C00930@ and perpendicular to the direction of the apical bonds are described as @E02174@. (See @A00546@ 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 @A00411@ of a pyramidal structure. The positions at or near the base of the pyramid, or the bonds linking those positions to the @C00930@ of the base are described as @B00600@. The apical bonds have also been called axial.
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2199 [Terms] [Paper]