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 " μ" with a superscript number denoting the number of metals bound to the bridging ligand is bound. μ2 is often denoted simply as μ.

Illustrative bridging ligands

Virtually all ligands are known to bridge, with the exception of amines and ammonia. Particularly common inorganic bridging ligands include

• OH−,
• O2−,
• S2−,
• SH−,
• NH2−
• NH2− (imido)
• N3− (nitrido)
• CO
• Halides
• Hydride
• Cyanide

Cyanide usually bridges via M-NC-M' linkages, unlike the other entries on this list. Many organic ligands form strong bridges between metal centers. Many common examples include derivatives of the above inorganic ligands (R = alkyl, aryl):

• OR−,
• SR−,
• NR2−
• NR2− (imido)
• P3− (phosphido)
• PR2− (phosphido, note the ambiguity with the preceding entry)
• PR2− (phosphinidino)

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 Carboxylate, PO43−, and the polyoxometallates. Several organophosphorus ligands have been developed that bridge pairs of metals, a well-known example being Ph2PCH2PPh2.

Examples

References