Ligand

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In biology, a ligand is an extracellular substance that binds to receptors. This rest of this article deals with ligands in the chemical sense.

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

Contents 1 Chemistry 1.1 Reactions 2 Geometry 3 Denticity 4 Isomerism 5 Spectroscopy 6 Common Ligands 7 See also

Chemistry

The central atom usually is usually cationic, which is stabilised by donation of negative charges from the ligands. Therefore, the ligand acts as a Lewis base by donating one or more electron pairs to the central atom, acting as a Lewis acid. Neutral or negatively-charged (anionic) centers are usually stabilised by donating electron density back to the ligand in a process known as back-bonding. If the directly-bonded ligands (the "inner-sphere" ligands) do not balance the charge of the central atom (the oxidation number), this may be done by simple ionic bonding with another set of counter ions (the "outer-sphere" ligands). In that case, the ligand complex is a complex ion (either cationic or anionic). The complex, along with its counter ions, is called a coordination compound.

Reactions

A common reaction between coordination complexes involving ligands are inner and outer sphere electron transfers. They are two different mechanisms of electron transfer redox reactions, largely defined by the late Henry Taube. In an inner sphere reaction, a ligand with two lone electron pairs acts as a bridging ligand, a ligand to which both coordination centres can bond. Through this, electrons are transfered from one centre to another.

Geometry

The inner-sphere ligands arrange themselves in a certain geometry—most commonly linear, tetrahedral, square planar, or octahedral. The arrangement is fixed for a given complex, but in some cases it is mutable by a reaction that forms another stable isomer.

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, whereas one with six would be octahedral.

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).

Denticity

Denticity is the property of a ligand defining the number of coordination sites to which it bonds. Ligands that 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 because they have free lone pairs on more than one atom. Ligands that bind to more than one site are termed polydentate or chelating (from the Greek for claw). For example, a ligand binding through two sites is bidentate and three sites is tridentate. A classic example of a polydentate ligand EDTA. 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. This increased stability or inertness is called the chelate effect. It is entropic in that more sites are used by less ligands and this leaves more unbonded molecules: a total increase in the number of molecules in solution and a corresponding increase in enropy.

Unlike polydentate ligands, these are ligands that can attach to the central atom in two places but not both. A good example of this is cyanide, CN^-, which can attach at either the carbon atom or the nitrogen. This form of structural isomerism—compounds composed of the same atoms but possess different bonds—is called linkage isomerism.

Isomerism

There exists two types of isomerism in coordination complexes, as in many other compounds. These are stereoisomerism and structural isomerism. The former occurs with the same bonds in different orientations relative to one another, the latter occurs when the bonds are themselves different.

There are two types of stereoisomerism: geometric isomerism and optical isomerism. The latter occurs when compounds have mirror images of themselves. The former occurs in octahedral and square planar complexes. When two ligands are oppositie each other they are said to be trans, when beside each other, cis. When an assortment of three ligands occurs in one of the planes of the polyhedron, they are said to be facial, or fac. If such an arrangement formas a plane cutting through the polyhedron, it is said to be meridional, or mer.

Linkage isomerism is only one of four major types of structural isomerism in coordination complexes, the others being ionisation isomerism, coordination isomerism, and solvate isomerism. Linkage isomerism, as described above, occurs with ambidentate ligands which can bind in more than one place. Ionisation isomerism describes the possible isomers occuring between the outer sphere and inner sphere. That is, the outer sphere ligands, when ions in solution, may be switched with inner sphere ions to produce an isomer in which the differences is which ions are bonded directly to the central atom and which are in solution. In solvate isomerism occurs when an inner sphere ligand is replaced by a solvent molecule. This is called hydrate isomerism when the solvent is water. The final type, coordination isomerism, occurs when a molecule of two complexes exchanges ligands.

Spectroscopy

Ligands are determinant on the spectra of complexes. Ligands can be either strongfield or weakfield. Strongfield ones donate electrons more strongly than weak-field ones. Strong-field ligands follow Aufbau's principle, whereas weak-field ligands follow Hund's rule. In crystal field theory, a strongfield ligand will increase the crystal field splitting parameter, Δ_o. This in turn will increase the likelihood of pairing of electrons in the central atom and will raise the energy needed for an electron transition, thus photons will only be absorbed at lower wavelengths. Weakfield ligands cause the opposite. When one wavelength of light in the visible spectrum in which these transitions occur is absorbed, its complementary colour is reflected. Thus, ligands are responsible for the vast array of colours seen in inorganic chemistry. The spectrochemical series describes ligands in a series ranging from weakfield to strongfield (or vice versa):

Strong Field Ligands > Weak Field Ligands

I^- < Br^- < S^2- < SCN^- < Cl^- < NO₃^- < N₃^- < F^- < OH^- < C₂O₄^2- < H₂O < NCS^- < CH₃CN < py (pyridine) < NH₃ < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO₂^- < PPh₃ < CN^- < CO

Common Ligands

See nomenclature.

Anionic Ligands

• F^-, fluoro
• Cl^-, chloro
• Br^-, bromo
• I^-, iodo
• OH^-, hydroxyl
• EDTA^4-, ethylenediaminetetraacetate
• CN^-, cyano

Neutral Ligands

• alkenes, compounds with carbon-carbon double bonds
• Benzene
• Cryptates
• Crown ethers
• Amines (R_nNH_2-n) and ammonia (NH₃, ammine)
• Water (aqua)
• Ethylenediamine
• Carbonyl
• Terpyridine
• Pyridine

Cationic Ligands

• Nitrosyl

See also

• Crystal field theory
• Ligand field theory
• Coordination chemistry
• Inorganic chemistryaf:Ligand

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