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In
coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
, a bridging ligand is a
ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex
organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
can serve as bridging ligands, so the term is usually restricted to small ligands such as
pseudohalide Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic ...
s or to ligands that are specifically designed to link two metals. In naming a complex wherein a single atom bridges two metals, the bridging ligand is preceded by the Greek letter mu, μ, with a subscript number denoting the number of metals bound to the bridging ligand. μ2 is often denoted simply as μ. When describing coordination complexes care should be taken not to confuse μ with η ('eta'), which relates to
hapticity In coordination chemistry, hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a l ...
. Ligands that are not bridging are called terminal ligands.


List of bridging ligands

Virtually all ligands are known to bridge, with the exception of
amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen Hydrogen is the chemical element wi ...
s and
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
. Common bridging ligands include most of the common anions. Many simple organic ligands form strong bridges between metal centers. Many common examples include organic derivatives of the above inorganic ligands (R = alkyl, aryl): , , , (imido), (phosphido, note the ambiguity with the preceding entry), (phosphinidino), and many more.


Examples

File:Ru-Cl.png, In this ruthenium complex ( (benzene)ruthenium dichloride dimer), two
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
ligands are terminal and two are μ2 bridging. File:Creutz-Taube-Ion.svg,
Pyrazine Pyrazine is a heterocyclic aromatic organic compound with the chemical formula C4H4N2. It is a symmetrical molecule with point group D2h. Pyrazine is less basic than pyridine, pyridazine and pyrimidine. It is a ''"deliquescent crystal or wax-li ...
is a bridging ligand in this diruthenium compound, called the Creutz–Taube complex. File:Mu3 compound.png, In the cobalt cluster , the ligand is triply bridging, although this aspect is typically not indicated in the formula. File:Fe3(CO)12lessFe-Fe.png, In triiron dodecarbonyl, two CO ligands are bridging and ten are terminal ligands. The terminal and bridging CO ligands interchange rapidly. File:Niobium-pentachloride-dimer-2D.png, In NbCl5, there are two bridging and eight terminal chloride ligands. File:Au6C(PPh3)6.png, The cluster features a μ6-
carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...
ligand, although again, the designator "μ" is not usually used. File:Aluminium-trifluoride-3D-polyhedra.png, In
rhenium trioxide Rhenium trioxide or rhenium(VI) oxide is an inorganic compound with the formula ReO3. It is a red solid with a metallic lustre that resembles copper in appearance. It is the only stable trioxide of the Group 7 elements ( Mn, Tc, Re). Prepara ...
, the oxide ligands are all μ2. These oxide ligands "glue" together the metal centres. File:Zirconium-tetrachloride-3D-balls-A.png, In the case of , there are both terminal and doubly bridging chloride ligands. File:Rhodium(II)-acetate-hydrate-dimer-from-xtal-1971-3D-balls.png, In
rhodium(II) acetate Rhodium(II) acetate is the coordination compound with the formula Rh2(AcO)4, where AcO− is the acetate ion (). This dark green powder is slightly soluble in polar solvents, including water. It is used as a catalyst for cyclopropanation of a ...
, the four acetate groups are bridging ligands. File:VO(HPO4)0.5H2O.tif, In , pairs of vanadium(IV) centers are bridged by water ligands.


Bonding

For doubly bridging (μ2-) ligands, two limiting representation are 4-electron and 2-electron bonding interactions. These cases are illustrated in main group chemistry by and . Complicating this analysis is the possibility of metal–metal bonding. Computational studies suggest that metal-metal bonding is absent in many compounds where the metals are separated by bridging ligands. For example, calculations suggest that lacks an iron–iron bond by virtue of a 3-center 2-electron bond involving one of three bridging CO ligands.


Polyfunctional ligands

Polyfunctional ligands can attach to metals in many ways and thus can bridge metals in diverse ways, including sharing of one atom or using several atoms. Examples of such polyatomic ligands are the oxoanions and the related
carboxylate In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge. Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...
s, , and the
polyoxometalate In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are ...
s. Several organophosphorus ligands have been developed that bridge pairs of metals, a well-known example being {{chem2, link=dppm, Ph2PCH2PPh2.


See also

* Bridging carbonyl


References

Coordination chemistry