In trigonal bipyramidal structures with a five-coordinate @[email protected], the stabilization achieved through a ligand changing its position from @[email protected] to apical (axial). The apicophilicity of an atom or a group is evaluated by either the energy difference between the @[email protected] (permutational isomers) containing the ligand in apical and @[email protected] positions or the energy barrier to permutational @[email protected] (see also @[email protected]). In general, the greater the @[email protected] and the stronger the π-electron-withdrawing properties of a ligand (as for Cl, F, CN), the higher is its apicophilicity. The notion of apicophilicity has been extended to four-coordinate bisphenoidal and three-coordinate T-shaped structures, which can be viewed as trigonal bipyramidal species where, respectively, one or two vertices are occupied by phantom @[email protected] (lone electron pairs).
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1923 [Terms] [Paper]