Sales Toll Free No: 1-855-666-7446

# Ligands

Top
 Sub Topics 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

"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

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

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

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

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

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.