coordination chemistry A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

, a bridging ligand is a

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex

organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides 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 A subscript or superscript is a character (such as a number or letter) that is set slightly below or above the normal line of type, respectively. It is usually smaller than the rest of the text. Subscripts appear at or below the baseline, wh ...

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 complex, 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 eta (letter), η ('eta ...

. 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 organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

s and

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

. 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 term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...

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-lik ...

is a bridging ligand in this diruthenium compound, called the Creutz–Taube complex. File:Mu3 compound.svg, 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 dodecacarbonyl, 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). Preparat ...

, 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, 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. BridgingLs

Bridge-terminal exchange

The interchange of bridging and terminal ligands is called bridge-terminal exchange. The process is invoked to explain the fluxional properties of

metal carbonyl A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against n ...

and metal isocyanide complexes. Some complexes that exhibit this process are cobalt carbonyl and cyclopentadienyliron dicarbonyl dimer: :Co₂(μ-CO)₂(CO)₆ Co₂(μ-CO)₂(CO)₄(CO)₂ :(C₅H₅)₂Fe₂(μ-CO)₂(CO)₂ (C₅H₅)₂Fe₂(μ-CO)₂(CO)₂ These dynamic processes, which are degenerate, proceed via an intermediate where the CO ligands are all terminal, i.e. .

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 anion, an ion with negative charge. Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,... ...

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.

References