Apicophilicity
Encyclopedia
Apicophilicity is the phenomenon in which electronegative
substituents of trigonal bipyramidal
pentacoordinate compounds
prefer to occupy apical positions (Lap).
The term "apicophilicity" was first proposed by Earl L. Muetterties in 1963 for the structural analysis of pentacoordinate phosphorus
fluoride
s by 19F NMR
. Since the apical bonding of a pentacoordinate typical (group
1, 2, 13-18) element compound consists of a 3-center-4-electron bond, in which the electron density
is localized on two apical substituent
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
ic structures in which the substituent occupies an apical position and an equatorial position (Leq). 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.
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 an apical and an equatorial position.
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
substituents of trigonal bipyramidal
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
pentacoordinate compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
prefer to occupy apical positions (Lap).
The term "apicophilicity" was first proposed by Earl L. Muetterties in 1963 for the structural analysis of pentacoordinate phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
s by 19F NMR
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
. Since the apical bonding of a pentacoordinate typical (group
Periodic table group
In chemistry, a group is a vertical column in the periodic table of the chemical elements. There are 18 groups in the standard periodic table, including the d-block elements, but excluding the f-block elements....
1, 2, 13-18) element compound consists of a 3-center-4-electron bond, in which the electron density
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
is localized on two apical substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
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
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
ic structures in which the substituent occupies an apical position and an equatorial position (Leq). Experimentally, instead of direct measurement of the energy difference, which is usually difficult to measure, the relative energy barriers for pseudorotation
Pseudorotation
The IUPAC defines pseudorotation as "a conformational change resulting in a structure that appears to have been produced by rotation of the entire initial molecule and is superimposable on the initial one, unless different positions are distinguished by substitution or isotopic labeling...
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
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
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 an apical and an equatorial position.