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 ('eta'), which relates to hapticity. Ligands that are not bridging, are called terminal ligands (see figure).
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.
In this ruthenium complex ((benzene)ruthenium dichloride dimer), two chloride ligands are terminal and two are μ2 bridging.
Pyrazine is a bridging ligand in this diruthenium compounds, called the Creutz-Taube complex.
In the cobalt cluster Co3(CO)9)Ct-Bu), the Ct-Bu ligand is triply bridging, although this aspect is typically not indicated in the formula.
In triiron dodecarbonyl, two CO ligands are bridging and ten are terminal ligands. The terminal and bridging CO ligands interchange rapidly.
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