Apicophilicity is the phenomenon in which

electronegative Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...

substituents of

trigonal bipyramidal In chemistry, a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular bipyramid. This is one geometry for which the bond angles surrounding the central atom are not identi ...

pentacoordinate compounds prefer to occupy apical (axial) positions (L_ap).IUPAC Gold Book
/ref> The term "apicophilicity" was first proposed by

Earl L. Muetterties Earl Muetterties (June 23, 1927 – January 12, 1984), was an Americans, American inorganic chemist born in Illinois, who is known for his experimental work with boranes, homogeneous catalysis, heterogeneous catalysis, Fluxional molecule, fluxion ...

in 1963 for the structural analysis of pentacoordinate

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...

s by ¹⁹F

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

. Since the apical bonding of a pentacoordinate typical (

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

1, 2, 13-18) element compound consists of a 3-center-4-electron bond, in which the

electron density Electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial variables and is typical ...

is localized on two apical

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

s, an arrangement in which electronegative substituents occupy apical positions is more stable. The apicophilicity of a substituent is defined as the difference in energy between two

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

ic structures in which the substituent occupies an apical position and an equatorial position (L_eq). Experimentally, instead of direct measurement of the energy difference, which is usually difficult to measure, the relative

energy barrier In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...

s for

pseudorotation In chemistry, a pseudorotation is a set of intramolecular movements of attached groups (i.e., ligands) on a highly symmetric molecule, leading to a molecule indistinguishable from the initial one. The International Union of Pure and Applied Chem ...

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. The apicophilicity of a substituent mainly depends on its electronegativity, but some other factors can also have an effect. A bulky substituent prefers equatorial positions, which are more distant from other substituents. A substituent with π-bonding ability also prefers equatorial positions. The use of multidentate ligands can also control the arrangement of 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. Image:Phosphorus-chloride-tetrafluoride-3D-balls.png, PClF₄ Image:Phosphorus-dichloride-trifluoride-3D-balls.png, PCl₂F₃ Image:Phosphorus-trichloride-difluoride-3D-balls.png, PCl₃F₂ Image:Phosphorus-tetrachloride-fluoride-3D-balls.png, PCl₄F

References

Chemical properties