Bridging ligand


In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds 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 character '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, which relates to hapticity. Ligands that are not bridging are called terminal ligands.

List of bridging inorganic ligands

Virtually all ligands are known to bridge, with the exception of amines and ammonia. Common inorganic bridging ligands include most of the common anions.
Bridging ligandNameExample
OH−hydroxide4+, see olation
O2−oxidedichromate|2−, see polyoxometalate
SH−hydrosulfidoCp2Mo22S2
NH2−amidoHgNH2Cl
N3−nitride4−, see metal nitrido complex
COcarbonylFe29, see metal carbonyl#Bridging carbonyls
Cl−chlorideNb2Cl10, see metal halide#Halide ligands
H−hydrideB2H6
CN−cyanideapprox. Fe718, see cyanometalate
PPh2−diphenylphosphidesee transition metal phosphido complexes

Many simple organic ligands form strong bridges between metal centers. Many common examples include organic derivatives of the above inorganic ligands : OR−, SR−, NR2−, NR2−, PR2−, PR2−, and many more.

Examples

Bonding

For doubly bridging ligands, two limiting representation are 4e and 2e bonding interactions. These cases are illustrated in main group chemistry by 2 and 2. 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 Fe29 lacks an Fe-Fe 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 CO32− and the related carboxylates, PO43−, and the polyoxometalates. Several organophosphorus ligands have been developed that bridge pairs of metals, a well-known example being Ph2PCH2PPh2.