A ligand is an ion or molecule which
exists independently of any complex
that it might form. In the complex [Co(NH3)6]3+,
the ammonia molecules, which can exist outside of the complex,
are acting as ligands. The complex is a combination of a Lewis
acid (the central metal atom/ion) and some Lewis
bases (the ligands).
The main way of classifying ligands is by the number of points
at which they are attached to, or bound to, the metal center.
This is the denticity. Many ligands
are monodentate, but some very important
ones are polydentate.
binding via oxygen
binding via nitrogen and oxygen)
(tetradentate, binding via nitrogen)
Ligands with more than one potential donor atom
are known as ambidentate, such
as the thiocyanate ion, NCS-, which can bind to the
metal center with either the nitrogen or sulphur atoms. Examples
of ambidentate ligands include NO2-/ONO-
(O and N), and SO32-/OSO22-
(O and S), where the first named atom refers to that which is
bonded to the metal center.
The existence of ambidentate ligands means that
there can be linkage isomerism:
exists in a red form and a yellow form. The red form has the
NO2- group bound by oxygen, when the ligand
is known as the nitrito group, and the yellow form has the NO2-
group bound by nitrogen, when the ligand is known as the nitro
are those polydentate ligands which can form a ring including
the metal atom. The complex formed is known as a chelate complex.
complex shows the ethylenediamine ligand chelating the
metal atom, and forming a 5-membered ring in the process.
Chelating ligands may often bind in more than
one arrangement, putting varying degrees of strain into the
ring formed. Five and six membered rings are often favoured
with saturated C and N based ligands, as in the example, as
the tetrahedral angles may be preserved, and with unsaturated
ligands as electron delocalization may be possible.
The distance of separation of the two donor atoms
in the chelating ligand, and the degree of strain this can introduce
into the ring in the complex, is expressed in terms of the bite
are those in which the donor atoms form a ring even before complex
formation. An example is the porphyrin ring, modified forms
of which are complexed with Fe at the O2 binding
site in haemoglobin, and with Mg in chlorophyll.
|The porphyrin macrocycle