Apicophilicity is the phenomenon in which electronegative substituents of trigonal bipyramidal pentacoordinate compounds prefer to occupy apical (axial) positions (Lap).
[1] The term "apicophilicity" was first proposed by Earl L. Muetterties in 1963 for the structural analysis of pentacoordinate phosphorus fluorides by 19F NMR.
Experimentally, instead of direct measurement of the energy difference, which is usually difficult to measure, the relative energy barriers for pseudorotation of isomers are used for determination of the apicophilicity scale.
Some experimental and theoretical studies have been made to measure relative apicophilicities for various substituents.
For example, a bidentate ligand with a five-membered ring structure greatly stabilizes the arrangement in which their two coordination sites occupy equatorial and an apical position.