Preferred Label : ion pair;
IUPAC definition : A pair of oppositely charged ions held together by coulomb attraction without formation
of a covalent bond. Experimentally, an ion pair behaves as one unit in determining
conductivity, kinetic behaviour, osmotic properties, etc. Following Bjerrum, oppositely
charged ions with their centres closer together than a distance: \[q \frac{8.36\times
10 {6}\ z { }\ z {-}}{\varepsilon_{{r}}\ T}\ {pm}\] are considered to constitute an
ion pair ('Bjerrum ion pair'). [z and z- are the charge numbers of the ions, and
ɛr is the relative permittivity (or dielectric constant) of the medium.] An ion pair,
the constituent ions of which are in direct contact (and not separated by an intervening
solvent or other neutral molecule) is designated as a 'tight ion pair' (or 'intimate'
or 'contact ion pair'). A tight ion pair of X sup /sup and Y sup /sup is symbolically
represented as X sup /sup Y sup /sup . By contrast, an ion pair whose constituent
ions are separated by one or several solvent or other neutral molecules is described
as a 'loose ion pair', symbolically represented as X sup class plus /sup Y sup
class minus /sup . The members of a loose ion pair can readily interchange with
other free or loosely paired ions in the solution. This interchange may be detectable
(e.g. by isotopic labelling) and thus afford an experimental distinction between tight
and loose ion pairs. A further conceptual distinction has sometimes been made between
two types of loose ion pairs. In 'solvent-shared ion pairs' the ionic constituents
of the pair are separated by only a single solvent molecule, whereas in 'solvent-separated
ion pairs' more than one solvent molecule intervenes. However, the term 'solvent-separated
ion pair' must be used and interpreted with care since it has also widely been used
as a less specific term for 'loose' ion pair.;
Origin ID : I03231;
See also
A pair of oppositely charged ions held together by coulomb attraction without formation
of a covalent bond. Experimentally, an ion pair behaves as one unit in determining
conductivity, kinetic behaviour, osmotic properties, etc. Following Bjerrum, oppositely
charged ions with their centres closer together than a distance: \[q \frac{8.36\times
10 {6}\ z { }\ z {-}}{\varepsilon_{{r}}\ T}\ {pm}\] are considered to constitute an
ion pair ('Bjerrum ion pair'). [z and z- are the charge numbers of the ions, and
ɛr is the relative permittivity (or dielectric constant) of the medium.] An ion pair,
the constituent ions of which are in direct contact (and not separated by an intervening
solvent or other neutral molecule) is designated as a 'tight ion pair' (or 'intimate'
or 'contact ion pair'). A tight ion pair of X sup /sup and Y sup /sup is symbolically
represented as X sup /sup Y sup /sup . By contrast, an ion pair whose constituent
ions are separated by one or several solvent or other neutral molecules is described
as a 'loose ion pair', symbolically represented as X sup class plus /sup Y sup
class minus /sup . The members of a loose ion pair can readily interchange with
other free or loosely paired ions in the solution. This interchange may be detectable
(e.g. by isotopic labelling) and thus afford an experimental distinction between tight
and loose ion pairs. A further conceptual distinction has sometimes been made between
two types of loose ion pairs. In 'solvent-shared ion pairs' the ionic constituents
of the pair are separated by only a single solvent molecule, whereas in 'solvent-separated
ion pairs' more than one solvent molecule intervenes. However, the term 'solvent-separated
ion pair' must be used and interpreted with care since it has also widely been used
as a less specific term for 'loose' ion pair.
