Liebermann testA *presumptive
test sometimes used for cocaine and
morphine. The Liebermann reagent
is a solution of potassium nitrite
(KNO 2 ) in sulphuric acid. With mor-
phine a black colour is produced; co-
caine gives a yellow colour.
Liebig, Justus von(1803–73) Ger-
man 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 com-
pounds by burning them and weigh-
ing the carbon dioxide and water
produced. With his students, he
analysed many compounds and was
extremely inÛuential in the develop-
ment of organic chemistry.
Liebig condenserA 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).
ligandAn ion or molecule that do-
nates a pair of electrons to a metal
atom or ion in forming a coordina-
tion *complex. Molecules that func-
tion 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 coor-
dinate to a central Cu2+ion. In the
tetrachloroplatinate(II) ion [PtCl 4 ]2–,
four Cl–ions are coordinated to a
central Pt2+ion. A feature of such lig-
ands 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 d-
orbitals in addition to their lone pair
of electrons and can produce com-
plexes 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-or-
bitals on the ligand. Carbon monox-
ide is the most important such
ligand, forming metal carbonyls (e.g.
Ni(CO) 4 ).
The examples given above are ex-
amples of monodentateligands (liter-
ally: ‘having one tooth’), in which
there is only one point on each lig-
and at which coordination can occur.
Some ligands are polydentate; i.e.
they have two or more possible coor-
dination points. For instance, 1,2-di-
aminoethane, H 2 NC 2 H 4 NH 2 , is a
bidentateligand, having two coordi-
nation points. Certain polydentate
ligands can form *chelates.
ligand-Üeld theoryAn extension
of *crystal-Üeld theory describing the
properties of compounds of transi-
tion-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 molecular-
orbital theory. Ligand-Üeld theory
was developed extensively in the
1930s. As with crystal-Üeld theory,
ligand-Üeld theory indicates that en-
ergy levels of the transition-metal
ions are split by the surrounding lig-
ands, as determined by *group
theory. The theory has been very suc-
cessful in explaining the optical,
spectroscopic, and magnetic proper-
ties of the compounds of transition-
metal and rare-earth ions.ligaseAny of a class of enzymes
that catalyse the formation of cova-
lent bonds using the energy released
by the cleavage of ATP. Ligases are
important in the synthesis and repair
of many biological molecules, includ-
ing DNA, and are used in genetic en-
gineering to insert foreign DNA into
cloning vectors.325 ligase
l