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A 3-center-2-electron bond, 3c-2e, is an electron deficient chemical bond where three atoms share two electrons, Wikipedia.
A bridging ligand, Wikipedia, 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 hydride, halide or pseudohalides or to ligands linking two metals.
Bridge bonding occurs when a Lewis base is held between a pair of vacant p or d orbital Lewis Acids. The system has two electrons which lead to the formation of a single bonding MO.
Borane, BH3, does not exist at room temperature because it dimerises to diborane. Hydride bridging bonds are found in diborane, B2H6., where the two central hydrogen atoms are simultaneously bonded to both boron atoms in 3c-2e bonds, Wikipedia:
The compound commonly called "trimethylaluminium, Al(CH3)3," is actually the dimer with the formula Al2(CH3)6, Wikipedia:
Halogen anion bridging bonds as in palladium[II] chloride, PdCl2:
In the ruthenium complex, (η6-C6H6)2Ru2Cl2(μ-Cl)2, two chloride ligands are terminal and two are μ2 bridging. The η in the beginning of the formula denotes the hapticity of the benzene ligands, Wikipedia:
Virtually all ligands are known to bridge, with the exception of amines and ammonia. Particularly common inorganic bridging ligands, from Wikipedia, are:
- Hydride, H–
- Halides, Cl–, Br– & I–
- Hydroxide, OH–
- Oxide, O2–
- Sulfide, S2–
- Hydrogen sulfide, SH–
- Carbon monoxide, CO