1802 by Joseph *Gay-Lussac. Thus the
law is also known as Gay-Lussac’s
law. An equation similar to that
given above applies to pressures for
ideal gases: p = p 0 (1 + t/273), a relationship known as Charles’ law of
pressures. See also gas laws.
cheddite Any of a group of high explosives made from nitro compounds
mixed with sodium or potassium
chlorate.
chelate An inorganic complex in
which a *ligand is coordinated to a
metal ion at two (or more) points, so
that there is a ring of atoms including the metal. The process is known
as chelation. Chelating agents are
classiÜed according to the number of
points at which they can coordinate.
A ligand such as diaminoethane
(H 2 N(CH 2 ) 2 NH 2 ), which can coordinate at two points, is said to be
bidentate. The angle made between
two bonds coordinating to a metal
atom is the bite angle of the ligand.
See also sequestration.
115
chemical bond
c
H 2 C
H 2 C
NH 2
Cu 2+
NH 2
NH 2
CH 2
CH 2
NH 2
Chelate
chelate effect The effect in which
a chelate complex is generally more
stable than the analogous complex
formed with monodentate ligands.
For example, the complex ion [Cu(en)
(OH 2 ) 4 ]
2+ is more stable than the complex ion [Cu(NH 3 ) 2 (OH 2 ) 4 ]
2+ . Here, en
denotes the bidentate ethylene diamine (1,2-diaminoethane) ligand.
The main cause of the chelate effect
is the effect of reaction entropy
when the complex is formed. Thus,
the reaction
[Cu(OH 2 ) 6 ]
2+ + en → [Cu(en)(OH 2 ) 4 +
2H 2 O
results in a net increase in the number of molecules (from 2 to 3). The
reaction
[Cu(OH 2 ) 6 ]
2+ + 2NH 3 →
Cu(NH 3 )2(OH 2 ) 4 + 2H 2 O
involves no net increase in the number of molecules. As a result, the
chelate reaction has a larger reaction
entropy and is more favourable.
cheletropic reaction A type of addition reaction in which a conjugated
molecule forms two single bonds
from terminal atoms of the conjugated system to a single atom on another molecule to give a cyclic
adduct. Cheletropic reactions are
types of *pericyclic reaction.
ChemDraw A widely used chemical drawing and modelling program
produced by CambridgeSoft.
A
• The CambridgeSoft website, giving
details about ChemDraw and associated
software
chemical bond A strong force of
attraction holding atoms together in
a molecule or crystal. Typically chemical bonds have energies of about
1000 kJ mol
–1 and are distinguished
from the much weaker forces between molecules (see van der waals’
force). There are various types.
Ionic (or electrovalent) bonds can be
formed by transfer of electrons. For
instance, the calcium atom has an
electron conÜguration of [Ar]4s
2 , i.e.
it has two electrons in its outer shell.
The chlorine atom is [Ne]3s
2 3p
5
, with
seven outer electrons. If the calcium
atom transfers two electrons, one to
each chlorine atom, it becomes a
Ca
2+ ion with the stable conÜguration of an inert gas [Ar]. At the same
time each chlorine, having gained
one electron, becomes a Cl
– ion, also
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law is also known as Gay-Lussac’s
law. An equation similar to that
given above applies to pressures for
ideal gases: p = p 0 (1 + t/273), a relationship known as Charles’ law of
pressures. See also gas laws.
cheddite Any of a group of high explosives made from nitro compounds
mixed with sodium or potassium
chlorate.
chelate An inorganic complex in
which a *ligand is coordinated to a
metal ion at two (or more) points, so
that there is a ring of atoms including the metal. The process is known
as chelation. Chelating agents are
classiÜed according to the number of
points at which they can coordinate.
A ligand such as diaminoethane
(H 2 N(CH 2 ) 2 NH 2 ), which can coordinate at two points, is said to be
bidentate. The angle made between
two bonds coordinating to a metal
atom is the bite angle of the ligand.
See also sequestration.
115
chemical bond
c
H 2 C
H 2 C
NH 2
Cu 2+
NH 2
NH 2
CH 2
CH 2
NH 2
Chelate
chelate effect The effect in which
a chelate complex is generally more
stable than the analogous complex
formed with monodentate ligands.
For example, the complex ion [Cu(en)
(OH 2 ) 4 ]
2+ is more stable than the complex ion [Cu(NH 3 ) 2 (OH 2 ) 4 ]
2+ . Here, en
denotes the bidentate ethylene diamine (1,2-diaminoethane) ligand.
The main cause of the chelate effect
is the effect of reaction entropy
when the complex is formed. Thus,
the reaction
[Cu(OH 2 ) 6 ]
2+ + en → [Cu(en)(OH 2 ) 4 +
2H 2 O
results in a net increase in the number of molecules (from 2 to 3). The
reaction
[Cu(OH 2 ) 6 ]
2+ + 2NH 3 →
Cu(NH 3 )2(OH 2 ) 4 + 2H 2 O
involves no net increase in the number of molecules. As a result, the
chelate reaction has a larger reaction
entropy and is more favourable.
cheletropic reaction A type of addition reaction in which a conjugated
molecule forms two single bonds
from terminal atoms of the conjugated system to a single atom on another molecule to give a cyclic
adduct. Cheletropic reactions are
types of *pericyclic reaction.
ChemDraw A widely used chemical drawing and modelling program
produced by CambridgeSoft.
A
• The CambridgeSoft website, giving
details about ChemDraw and associated
software
chemical bond A strong force of
attraction holding atoms together in
a molecule or crystal. Typically chemical bonds have energies of about
1000 kJ mol
–1 and are distinguished
from the much weaker forces between molecules (see van der waals’
force). There are various types.
Ionic (or electrovalent) bonds can be
formed by transfer of electrons. For
instance, the calcium atom has an
electron conÜguration of [Ar]4s
2 , i.e.
it has two electrons in its outer shell.
The chlorine atom is [Ne]3s
2 3p
5
, with
seven outer electrons. If the calcium
atom transfers two electrons, one to
each chlorine atom, it becomes a
Ca
2+ ion with the stable conÜguration of an inert gas [Ar]. At the same
time each chlorine, having gained
one electron, becomes a Cl
– ion, also
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