Ligand Did You Mean Ligand?

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For ligand in the biochemical sense, see Ligand (biochemistry).

In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through coordinate covalent bond. An array of such ligands around a centre is termed a complex.

In biology, a ligand is an extracellular substance that binds to receptors.

The central atom usually has a positive charge which is stabilised by donation of negative charge from the ligands. Neutral or negatively charged centres are also known, usually stabilised by donating electron density back to the ligand in a process known as "back-bonding". The charge on the central atom constrains the number of ligands that may be bonded, since each type of ligand donates a characteristic number of electrons and there is a requirement for overall neutrality. If the directly-bonded ligands (the "inner-sphere" ligands) do not balance the charge, this may be done by purely ionic interaction with another set of ions (the "outer-sphere" ligands).

The inner-sphere ligands arrange themselves in a certain geometry, fixed for a given complex but in some cases mutable by reaction to another stable isomer. Ligands which bind to the central atom through more than one site of their own are termed polydentate; a ligand binding through two sites, for example, is bidentate.

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Ligand arrangements

The ligand geometries are named and described as if the central atom were in the middle of a polyhedron, and the corners of that shape were the locations of the ligands. For example, a complex with four regularly distributed ligands would be described as tetrahedral, while one with six would be hexahedron.

The polyhedra need not be regular: there are other possible geometries, such as square pyramidal (four ligands equally distributed in a plane, and one ligand normal to this plane).

Polydentate ligands

Ligands which only bond to the central atom through one site are termed monodentate. Some ligand molecules are able to bind to the metal ion through multiple sites, since they have free lone pairs on more than one atom; these are called polydentate. A ligand with two coordination sites is called bidentate. EDTA is a classic example of a polydentate ligand - it is able to bond through six sites, completely surrounding the central atom. A scorpionate ligand is an example of a tridentate ligand. Complexes of polydentate ligands are called chelated complexes; they tend to be more stable than monodentate complexes as it is necessary to break all of the bonds to the central atom for the ligand to be displaced.

Ambidentate Ligands

These are ligands that can attach to the central atom in two places but not both, unlike polydentate ligands. A good example of this is CN- which can attach at either the Carbon atom or the Nitrogen but not both.

Common ligands

F^-
Cl^-
Br^-
I^-
CO
RC:CR - compounds with carbon-carbon double bonds
Benzene
Cryptates
Crown ethers
OH^-
Amines (RNH₂) and ammonia (NH₃)

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Contents

Ligand arrangements

Polydentate ligands

Ambidentate Ligands

Common ligands

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