In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex.
|Neutral ligands which are named as such:|
(C6H5)3P triphenyl phosphine
(C2H5)3 P triethyl phosphine
C5H5N pyridine (py)
NH2 – NH2 hydrazine
|Neutral ligands which are given special name:|
Ligands can be classified as monodentate or polydentate ligands (viz. Bidentate, tridentate) depending on the number of ligands donor atoms that attach to the metal ion or atom.
Monodentate ligands are also called “one-toothed“ because they bite the metal atom only in one place.
Lewis base which donates two lone pairs of electrons to the central metal atom is known as bidentate ligands.
|Acetylacetonate ion (acac)|
|Oxalate ion (ox)|
propylenediamine (pn) or 1, 2-diaminopropane
butylenediamine (bn) or 2, 3-diaminopropane
isobutylenediamine (i- bn)
bipyridine or bipyridyl (bpy or bipy)
1, 10-phenanthroline Or o-phenanthroline (o-phen or phen)
o-phenylene bis dmethylarisine (diars)
Note: The ligands like CO32-, CH3COO-, NO2-, NO3-, SO32-, SO42-, NH2- NH2, O2N2, etc. have two donor atoms but in general, they act as monodentate ligands. They form either four or three-membered rings including metal cation or atom and there will be repulsion between donor atoms and their electron pairs which causes the ring to be strained and hence unstable.
Four Membered ring
These ligands may behave as bidentate ligands when the size of the metal cation is large like lanthanoids. For example, NO3- behaves as a bidentate ligand in [Ce(NO3)6]2– in which the coordination number of Ce4+ is 12.
The ligands that bond to metal cation or atom through electron pairs present on more than one donor atoms are called multidentate or polydentate ligands (many toothed ligands).
Tridentate ligands have three lone pairs of electrons to the central metal atom or ion.
Some important examples of tridentate ligands are shown below:
Molecules with four donor atoms are called tetradentate.
Some important examples of tetradentate ligands are given below:
Molecules with five donor atoms are called Pentadentate.
Important examples of pentadentate ligands are given below:
Molecules with Six donor atoms are called Hexadentate Ligands.
The most important examples of hexadentate ligands is:
The monodentate ligands which have two or more different donor atoms can coordinate to a metal cation through either of the two different atoms. These ligands are called ambidentate ligands.
When ambidentate ligand coordinates to the metal cation through either of the two donor atom, two different compounds are obtained which are called linkage isomers. Examples of ambidentate ligands are:
none of these has yet produced linkage isomers.NO -2 ligand, for example, can coordinate to a metal cation through either N or O atom as shown in Fig. 6.
Similarly, SCN– can coordinate with metal cation either through S or N atom as shown below:
M ←SNC– M ← NCS–
S-atom of SCN– N-atom of SCN–
Coordinated to metal ion coordinated to metal ion
Chelating ligands (chelates) – Ligands that have two or more points of attachment to the metal atom or ion such as Bidentate, tridentate, tetradentate, pentadentate, hexadentate (EDTA) ligands.
Example: [Cr(ox)3]3- has oxalate ion as the chelating ligand.
Example: [Ca(EDTA)]2- has EDTA ion as the chelating ligand.
The ligands in which one or two different donor atoms have atleast two pairs of electrons and share these electron pairs with two metal ions or atoms (one pair with one atom or ion) simultaneously are called bridging ligands.
The interaction of bridging ligand with metal ions or atoms can be represented
as M ← : L : → M.
The monodentate ligands having two lone pair of electrons on one donor atom which act as bridging ligands are The monodentate ligands having two lone pairs on two different atoms (one pair on one atom) are