Incoordination chemistry, abridging ligand is aligand that connects two or more atoms, usually metal ions.^[1] The ligand may be atomic or polyatomic. Virtually all complexorganic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such aspseudohalides 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 lettermu, μ,^[2] with asubscript 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 withη ('eta'), which relates tohapticity. Ligands that are not bridging are calledterminal ligands.

Virtually all ligands are known to bridge, with the exception ofamines andammonia.^[3] 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):OR⁻,SR⁻,NR−2,NR²⁻ (imido),PR−2 (phosphido, note the ambiguity with the preceding entry),PR²⁻ (phosphinidino), and many more.

• Compounds and complexes with bridging ligands
• 
  In this ruthenium complex ((benzene)ruthenium dichloride dimer), twochloride ligands are terminal and two are μ₂ bridging.
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  Pyrazine is a bridging ligand in this diruthenium compound, called theCreutz–Taube complex.
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  In the cobalt clusterCo₃(CO)₉(C^tBu), theC^tBu ligand is triply bridging, although this aspect is typically not indicated in the formula.
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  Intriiron dodecacarbonyl, two CO ligands are bridging and ten are terminal ligands. The terminal and bridging CO ligands interchange rapidly.
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  InNbCl₅, there are two bridging and eight terminal chloride ligands.
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  The cluster[Au₆C(PPh₃)₆]²⁺ features a μ₆-carbide ligand, although again, the designator "μ" is not usually used.
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  Inrhenium trioxide, the oxide ligands are all μ₂. These oxide ligands "glue" together the metal centres.
• 
  In the case ofZrCl₄, there are both terminal and doubly bridging chloride ligands.
• 
  Inrhodium(II) acetate, the four acetate groups are bridging ligands.
• 
  InVO(HPO₄)·0.5H₂O, pairs of vanadium(IV) centers are bridged by water ligands.^[4]

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[Me₂Al(μ₂-Cl)]₂ and[Me₂Al(μ₂-Me)]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 thatFe₂(CO)₉ lacks an iron–iron bond by virtue of a3-center 2-electron bond involving one of three bridging CO ligands.^[5]

The interchange of bridging and terminal ligands is calledbridge-terminal exchange. The process is invoked to explain the fluxional properties ofmetal carbonyl andmetal isocyanide complexes.^[6] Some complexes that exhibit this process arecobalt carbonyl andcyclopentadienyliron 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.,(CO)₄Co−Co(CO)₄ and (C₅H₅)(CO)₂Fe−Fe(CO)₂C₅H₅.

Bridge-terminal exchange is common formetal complexes of borohydride where the M-H-B and B-H_terminal sites interchange with low barriers.^[7]

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 oxoanionsCO2−3 and the relatedcarboxylates,PO3−4, and thepolyoxometalates. Several organophosphorus ligands have been developed that bridge pairs of metals, a well-known example beingPh₂PCH₂PPh₂.

• Bridging carbonyl

• Coordination chemistry

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