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 letter 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. Virtually all ligands are known to bridge, with the exception of amines and ammonia. 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): , , , (imido), (phosphido, note the ambiguity with the preceding entry), (phosphinidino), and many more. File:Ru-Cl.png|In this ruthenium complex ([[(cymene)ruthenium dichloride dimer|(benzene)ruthenium dichloride dimer]]), two [[chloride]] ligands are terminal and two are μ2 bridging. File:Creutz-Taube-Ion.svg|[[Pyrazine]] is a bridging ligand in this diruthenium compound, called the [[Creutz–Taube complex]]. File:Mu3 compound.svg|In the cobalt cluster {{chem2|Co3(CO)9(C^{''t''}Bu)}}, the {{chem2|link=Alkylidyne complex|C^{''t''}Bu}} ligand is triply bridging, although this aspect is typically not indicated in the formula. File:Fe3(CO)12lessFe-Fe.png|In [[triiron dodecacarbonyl]], two CO ligands are bridging and ten are terminal ligands. The terminal and bridging CO ligands interchange rapidly. File:Niobium-pentachloride-dimer-2D.png|In [[Niobium pentachloride|NbCl5]], there are two bridging and eight terminal chloride ligands.