Ligand
This article describes the meaning of the term ligand in complex chemistry. For the meaning in biochemistry, see ligand (biochemistry).
In complex chemistry (as well as in organometallic chemistry and bioinorganics), a ligand (lat. ligare = "to bind") is an atom or molecule that can bind ("coordinate") to a central metal ion via a coordinative bond (obsoletely also "dative bond"). The coordinative bond comes about through the Lewis character of the binding partners involved: In many complexes, the ligands are Lewis bases (electron pair donors) and the metal ions are Lewis acids (electron pair acceptors).
The special feature of the coordinative bond is that (in contrast to the classical covalent bond) both bond electrons are provided by the ligand or metal ion. Complex compounds are, for example, the red blood lye salt (potassium hexacyanidoferrate(III)), in which the central iron(III) ion is surrounded by six ligands, in this case cyanide ions.
Examples from bioinorganics are heme and chlorophyll: here the four nitrogen atoms in the porphyrin skeleton act as a multidentate ligand for the central iron or magnesium ion.
The description of the metal-ligand bond as a purely donative bond is highly simplistic. Especially in the case of heavy transition metal ions, the bonds usually have a high classical covalent component (for example, in the case of hydride ligands). Also, back-bonding effects from the metal to the ligand often play a crucial role in the bond strength (binding energy). This is the case for many ligands with π-systems, since the metal centers can shift electron density to the π*-orbitals; examples of such ligands are CO, whose complexes are called metal carbonyls, and ethene.
According to the CBC formalism, ligands can be classified as L-,X- or Z-type. L-type ligands provide one pair of electrons for binding. These are Lewis bases such as phosphanes or the carbonyl molecule. In contrast, neutrally charged X-type ligands can only provide one electron for bonding to the central atom, so an electron from the metal atom is required for bonding. Examples of ligands of this type are the clorido or cyanate ligand. On the other hand, if the ligand is a Lewis acid, it is classified as a Z-type ligand. These are electron deficient compounds such as AlCl3 or BR3, so that two electrons must be provided by the metal atom for bonding.
In organometallic chemistry, ligands have always been used to keep active metal centers stable in solution and to prevent them from agglomerating into metallic colloids (nanoparticles). These soluble metal complexes can be optimized into highly active catalysts for homogeneous catalysis by appropriate choice of the electronic and steric properties of the ligands.
Protoporphyrin IX as a ligand for iron ions in heme b.
See also
- Chelate ligand
- Non-innocent ligand
- Ligand abbreviations
- Ligand Field Theory
- Ligand cone angle
- Complexation reaction
- Phosphane ligands
- Tolman Electronic Parameters
Questions and Answers
Q: What is a ligand?
A: A ligand is a biochemical substance that binds to a biological molecule in order to make it perform a function.
Q: How does ligand binding affect the receptor protein?
A: Ligand binding alters the shape of the receptor protein, which in turn changes the protein's behavior.
Q: How does ligand binding occur?
A: Ligand binding occurs through intermolecular forces such as ionic bonds, hydrogen bonds, and van der Waals forces.
Q: Is ligand binding reversible?
A: Yes, ligand binding is usually reversible and can be dissociated.
Q: What are some examples of ligands?
A: Ligands include substrates, inhibitors, activators, and neurotransmitters.
Q: What is the tendency or strength of binding called?
A: The tendency or strength of binding is called affinity.
Q: What is the specific function of a ligand in relation to target proteins?
A: In a narrower sense, a ligand is a signal-triggering molecule that binds to a specific site on a target protein.
