Liebermann test A *presumptive
test sometimes used for cocaine and
morphine. The Liebermann reagent
is a solution of potassium nitrite
(KNO 2 ) in sulphuric acid. With morphine a black colour is produced; cocaine gives a yellow colour.
Liebig, Justus von (1803–73) German organic chemist who worked at
Gessen in Frankfurt. Liebig was the
Ürst to recognise that two different
chemical compounds can have the
same formula. He also developed a
method of analysing organic compounds by burning them and weighing the carbon dioxide and water
produced. With his students, he
analysed many compounds and was
extremely inÛuential in the development of organic chemistry.
Liebig condenser A laboratory
condenser having a straight glass
tube surrounded by a coaxial glass
jacket through which cooling water
is passed. The device is named after
the German organic chemist Justus
von Liebig (1803–73).
ligand An ion or molecule that donates a pair of electrons to a metal
atom or ion in forming a coordination *complex. Molecules that function as ligands are acting as Lewis
bases (see acid). For example, in the
complex hexaquocopper(II) ion
[Cu(H 2 O) 6 ]
2+ six water molecules coordinate to a central Cu
2+ ion. In the
tetrachloroplatinate(II) ion [PtCl 4 ]
2–
,
four Cl
– ions are coordinated to a
central Pt
2+ ion. A feature of such ligands is that they have lone pairs of
electrons, which they donate to
empty metal orbitals. A certain class
of ligands also have empty p- or dorbitals in addition to their lone pair
of electrons and can produce complexes in which the metal has low
oxidation state. A double bond is
formed between the metal and the
ligand: a sigma bond by donation of
the lone pair from ligand to metal,
and a pi bond by back donation of
electrons on the metal to empty d-orbitals on the ligand. Carbon monoxide is the most important such
ligand, forming metal carbonyls (e.g.
Ni(CO) 4 ).
The examples given above are examples of monodentate ligands (literally: ‘having one tooth’), in which
there is only one point on each ligand at which coordination can occur.
Some ligands are polydentate; i.e.
they have two or more possible coordination points. For instance, 1,2-diaminoethane, H 2 NC 2 H 4 NH 2 , is a
bidentate ligand, having two coordination points. Certain polydentate
ligands can form *chelates.
ligand-Üeld theory An extension
of *crystal-Üeld theory describing the
properties of compounds of transition-metal ions or rare-earth ions in
which covalent bonding between the
surrounding molecules (see ligand)
and the transition-metal ions is taken
into account. This may involve using
valence-bond theory or molecularorbital theory. Ligand-Üeld theory
was developed extensively in the
1930s. As with crystal-Üeld theory,
ligand-Üeld theory indicates that energy levels of the transition-metal
ions are split by the surrounding ligands, as determined by *group
theory. The theory has been very successful in explaining the optical,
spectroscopic, and magnetic properties of the compounds of transitionmetal and rare-earth ions.
ligase Any of a class of enzymes
that catalyse the formation of covalent bonds using the energy released
by the cleavage of ATP. Ligases are
important in the synthesis and repair
of many biological molecules, including DNA, and are used in genetic engineering to insert foreign DNA into
cloning vectors.
325
ligase
l
www.AzShimi.ir www.AzShimi.com
test sometimes used for cocaine and
morphine. The Liebermann reagent
is a solution of potassium nitrite
(KNO 2 ) in sulphuric acid. With morphine a black colour is produced; cocaine gives a yellow colour.
Liebig, Justus von (1803–73) German organic chemist who worked at
Gessen in Frankfurt. Liebig was the
Ürst to recognise that two different
chemical compounds can have the
same formula. He also developed a
method of analysing organic compounds by burning them and weighing the carbon dioxide and water
produced. With his students, he
analysed many compounds and was
extremely inÛuential in the development of organic chemistry.
Liebig condenser A laboratory
condenser having a straight glass
tube surrounded by a coaxial glass
jacket through which cooling water
is passed. The device is named after
the German organic chemist Justus
von Liebig (1803–73).
ligand An ion or molecule that donates a pair of electrons to a metal
atom or ion in forming a coordination *complex. Molecules that function as ligands are acting as Lewis
bases (see acid). For example, in the
complex hexaquocopper(II) ion
[Cu(H 2 O) 6 ]
2+ six water molecules coordinate to a central Cu
2+ ion. In the
tetrachloroplatinate(II) ion [PtCl 4 ]
2–
,
four Cl
– ions are coordinated to a
central Pt
2+ ion. A feature of such ligands is that they have lone pairs of
electrons, which they donate to
empty metal orbitals. A certain class
of ligands also have empty p- or dorbitals in addition to their lone pair
of electrons and can produce complexes in which the metal has low
oxidation state. A double bond is
formed between the metal and the
ligand: a sigma bond by donation of
the lone pair from ligand to metal,
and a pi bond by back donation of
electrons on the metal to empty d-orbitals on the ligand. Carbon monoxide is the most important such
ligand, forming metal carbonyls (e.g.
Ni(CO) 4 ).
The examples given above are examples of monodentate ligands (literally: ‘having one tooth’), in which
there is only one point on each ligand at which coordination can occur.
Some ligands are polydentate; i.e.
they have two or more possible coordination points. For instance, 1,2-diaminoethane, H 2 NC 2 H 4 NH 2 , is a
bidentate ligand, having two coordination points. Certain polydentate
ligands can form *chelates.
ligand-Üeld theory An extension
of *crystal-Üeld theory describing the
properties of compounds of transition-metal ions or rare-earth ions in
which covalent bonding between the
surrounding molecules (see ligand)
and the transition-metal ions is taken
into account. This may involve using
valence-bond theory or molecularorbital theory. Ligand-Üeld theory
was developed extensively in the
1930s. As with crystal-Üeld theory,
ligand-Üeld theory indicates that energy levels of the transition-metal
ions are split by the surrounding ligands, as determined by *group
theory. The theory has been very successful in explaining the optical,
spectroscopic, and magnetic properties of the compounds of transitionmetal and rare-earth ions.
ligase Any of a class of enzymes
that catalyse the formation of covalent bonds using the energy released
by the cleavage of ATP. Ligases are
important in the synthesis and repair
of many biological molecules, including DNA, and are used in genetic engineering to insert foreign DNA into
cloning vectors.
325
ligase
l
www.AzShimi.ir www.AzShimi.com