apical (basal, equatorial)

https://doi.org/10.1351/goldbook.A00415
In trigonal bipyramidal structures (e.g. a five-coordinate trigonal bipyramid with phosphorus as @[email protected]) the term apical refers to the two positions that are collinear with the @[email protected] or to the bonds linking these positions to the @[email protected] The three equivalent bonds (or positions) in a plane passing through the @[email protected] and perpendicular to the direction of the apical bonds are described as @[email protected] (See @[email protected] 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 @[email protected] of a pyramidal structure. The positions at or near the base of the pyramid, or the bonds linking those positions to the @[email protected] of the base are described as @[email protected] The apical bonds have also been called axial.
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Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2199 [Terms] [Paper]