bridging ligand
/ the Greek letter mu is the bridging symbol /
Most ligands hold one metal. But some can hold two at once — like a person reaching out to shake hands with two people standing apart, joining them. A bridging ligand links two (or more) metal centers together, acting as a bridge between them.
Precisely, a bridging ligand is one that binds to two or more metal atoms simultaneously, connecting them into a larger structure such as a dinuclear complex or a metal cluster. It does this either by using two different donor atoms (each reaching a different metal) or by using a single atom that has more than one lone pair to share. Common bridging ligands include hydroxide and oxide (a single O reaching two metals), chloride and other halides, carbonate, and cyanide (which bridges through both its carbon and its nitrogen). In formulas and names the bridge is marked with the Greek letter mu, as in mu-hydroxo or mu-chloro; a subscript on mu (mu-3, mu-4) tells how many metals the one ligand bridges. The same chemical species — chloride, say — can be a simple terminal ligand on one complex and a bridge in another.
Bridging ligands are how chemistry builds beyond a single metal center: they assemble dinuclear complexes, polynuclear clusters, and extended solids, and they provide the pathway for electron transfer between metals in the inner-sphere mechanism (an electron hops across the bridge from one metal to the other). Many biological and industrial systems rely on bridged metal pairs — iron-oxo bridges in enzymes, for instance. Recognizing a bridge is essential to reading the structure of any complex with more than one metal.
In the dinuclear ion [(NH3)5Co-OH-Co(NH3)5]5+, a single hydroxide ligand bridges the two cobalt atoms, named mu-hydroxo. The one oxygen donates a lone pair to each cobalt, holding the two halves together like a hinge.
A single hydroxide can hinge two cobalt centers together as a mu-bridge.
Bridging and chelating are different jobs: a chelating ligand grips one metal at two points, a bridging ligand connects two metals. The same kind of ligand can do either depending on the geometry — do not assume two donor atoms always means chelation.