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A ligand is an entity that binds strongly to a central species. In coordination chemistry a ligand is a molecule or ion carrying suitable donor groups capable of binding to a central atom. The term ligand first appeared early in the twentieth century and achieved popular use by mid century. In past decades the development of numerous ligands that provide good catalysts for various reactions.

Ligand design has played an important role in the development of new reactivity patterns at metal centers. Typically ligands are chosen on a metal by metal basis and their steric and electronic properties are tailored to the particular metal. Ligand is the general term applied to any chemical that is capable of interacting with or binding to a membrane protein or membrane receptor.

Ligand Definition

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"A ligand can be defined as a specific substance that is capable of binding and forming a thermodynamically stable complex with other bio molecules through either ionic and hydrogen bonding or Van der Waals forces to perform a specific biologically important function."
The word ligand is derived from the Latin word ligare which means to bind. A ligand is a specific signal inducing molecule such as an inhibitor, activator, substrate or neurotransmitter that binds to macromolecules such as proteins irreversibly via intermolecular forces. The strength of binding is referred to as affinity.

Naming Ligands

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In naming the coordination sphere the ligands are named first and then the central metal ion. Some simple rules for naming the ligands are given below.
  • Anionic ligands ends in -o
F- Fluoro
NO2-
Nitro
SO32-
Sulfito OH-
Hydroxo
Cl-
Chloro
ONO-
Nitrito
SO42-
Sulfato
CN-
Cyano
Br-
Bromo
NO3-
Nitrato
S2O32-
Thiosulfato
NC-
Isocyano
I-
Iodo
CO32-
Carbonato
ClO3-
Chlorato
SCN-
thiocyanato
O2- Oxo
C2O42-
Oxalato
CH3COO-
acetato
NCS-
isothiocyanato
  • Neutral ligands are named as the neutral molecule.
C2H4
ethylene
(C6H5)3P
triphenylphosphine
NH2CH2CH2NH2
ethylenediamine
CH3NH2
methylamine
  • There are special names for some neutral ligands.
H2O
aqua
NH3
amine
CO
Carbonyl
NO
Nitrosyl
  • Cationic ligands end in -ium.
NH2NH3+ - hydrazinium
  • Ambidentate ligands are indicated by
  1. Using special names for the two forms for example nitro and nitrito for -NO2- and -ONO-
  2. Placing the symbol of the coordinating atom in front of the name of the ligand for example S-thiocyanato and N-thiocyanato for -SCN- and NCS-.
  • Bridging ligands are indicated by placing a $\mu$ before the name of the ligand.

Types of Ligands

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Ligands have been classified in two ways. Classification based on donor and acceptor properties of the ligands: Ligands having one lone pair of electrons. These ligands are of the following two types.
  1. First type includes such ligands which have vacant $\pi$-electrons from the metals ion in low oxidation state. The main examples of such ligands are CO, CN, isocyanides, NO, R3P, R3As, $\alpha$, $\alpha$-dipyridyl, o-phenanthroline and unsaturated organic molecules.
  2. Second type includes such ligands which have no vacant orbitals to receive back donated electrons from the metal. Examples of this type are H2O, NH3, F- etc.

Ligand Binding

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The binding of the ligand over a period of time or the ligands dissociation over time is measured. A binding assay only gives information about a ligands binding to binding sites. The system to be assayed consists of a concentration of binding sites and a concentration of the ligand, which is either bound to these binding sites or free in solution.

Binding of a ligand might even have a very non specific and artificial basis. For example, the ligand might bind to fragments of receptor proteins, which contain the binding site, or it may bind because of adsorption to glass surfaces, biological membranes or proteins which do not have any sign of a specific binding site for the ligand.

Neutral Ligands

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Ligands are Lewis bases which donates electron pair and forms coordinate bonds with the metal atom. Ligands may be classified into cationic, anionic or neutral depending upon the charge present.

Neutral ligands do not carry any change but contains one or more unshared pair of electron. Some of the examples for neutral ligands are given below.
  1. NH3 (amine)
  2. H2O (aqua)
  3. H2NCH2CH2NH2 (ethylene diamine)
  4. CO (carbonyl)

Ligand Efficiency

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The concept of ligand efficiency is useful for qualifying the potential of a fragment for lead development. Ligand efficiency is defined as the free energy if binding divided by the number of non hydrogen atoms.

Ligand efficiency (LE) = $\frac{\Delta G}{N_{non\ hydrogen\ atoms}}$

The concept of ligand efficiency can be extended to include other molecular properties relevant for lead optimization when one would like to control these properties while retaining potency.

Spectrochemical Series of Ligands

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The ligands are arranged in the order of their increasing splitting ability. This series is called the spectrochemical series (since the values of splitting abilities are obtained from the spectra of their complexes).

I- < Br- < S2- < SCN- < Cl- < NO3- < F- < OH- < C2O42- < H2O < NH3 < en < bipy < o-phen < NO2- < CN- < CO

The first ligands in the above are known as weak field ligands. The last few ligands are called the strong field ligands. This order of increasing field strength of the common ligands is independent of the nature of the central metal ion and the geometry of the complex.

Examples of Ligands


The atom in the ligand which actually donates the electron pair is called a donor atom or coordinating atom. The common donor atoms are nitrogen, oxygen, sulfur and halogens. A ligand may be neutral such as ammonia (NH3), water (H2O) etc, or a negatively charged ion such as F-, Cl-, Br-, I-, CN-, SCN-, NH2- etc.

Examples of unidentate ligands are H2O, NH3, F-, Cl-, Br-, I-, SCN-, CN- etc. A unidentate ligand generally form only one coordinate bond with the metal ion. Examples of the ligands containing two donor atoms are ethylene diamine and dimethylglyoximato etc.