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.
| monodentate | bidentate | tridentate | polydentate |
| Ammine, NH3Aqua, OH2
hydroxo, OH– Chloro, Cl– |
acetylacetonato, (CH3COCHCOCH3)–, binding via oxygenbipyridene
ethylenediamine, H2NCH2CH2NH2 oxalato, C2O42- |
diethylenetriamine, NH(CH2CH2NH2)2 | EDTA4-, (sexidentate, binding via nitrogen and oxygen)triaminoethylamine, N(CH2CH2NH2)3, (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: [Co(NO2)(NH3)5]2+ 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 group.
Chelating ligands are those polydentate ligands which can form a ring including the metal atom. The complex formed is known as a chelate complex.
|
The 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 distance.
Macrocyclic ligands 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 |
 |