Bridging ligand

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 superscript 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. Ligands that are not bridging, are called terminal ligands (see figure).

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.

Many simple organic ligands form strong bridges between metal centers. Many common examples include organic derivatives of the above inorganic ligands (R = alkyl, aryl): OR⁻, SR⁻, NR₂⁻, NR²⁻ (imido), PR₂⁻ (phosphido, note the ambiguity with the preceding entry), PR²⁻ (phosphinidino), and many more.