bridging ligand electron transfer
/ bridging ligand /
If inner-sphere electron transfer is two people passing a coin along a shared umbrella handle, the bridging ligand is the handle itself. It is the single atom or group that, for a brief moment, holds onto both metals at once and gives the electron a path to travel. Without a good bridge, the inner-sphere route simply cannot operate, so understanding what makes a ligand a good bridge is understanding the rate of these reactions.
A bridging ligand is one that donates lone pairs to two metal centres simultaneously, linking them into a bridged binuclear complex during the electron-transfer step. To do its job well it must have at least two donor sites or a donor with extra lone pairs (chloride, bromide, hydroxide, azide, thiocyanate, and conjugated organics like pyrazine all qualify), and it helps enormously if the bridge can conduct the electron. Here a key distinction appears. In a chemical mechanism the bridge is briefly reduced — it actually accepts the electron into its own orbitals and then passes it on, like a stepping stone, which works for ligands with accessible empty orbitals such as conjugated organics. In a resonance mechanism the electron tunnels straight through the bridge in one step without ever truly residing on it, the bridge merely providing an electronic pathway. Bridges that offer a continuous, conjugated set of orbitals (such as 4,4'-bipyridine spanning a long distance) can carry electrons surprisingly far.
The bridging ligand explains why inner-sphere rates can swing over many orders of magnitude depending on which bridge you choose. Azide bridges, for instance, often transfer electrons far faster than the otherwise similar isothiocyanate, because their orbital pathways differ. This sensitivity to the bridge is itself the clearest sign that a reaction goes by the inner-sphere route rather than outer-sphere. The same bridged binuclear motif, frozen rather than fleeting, also gives the famous mixed-valence compounds such as the Creutz-Taube ion, where two ruthenium centres in different formal oxidation states are joined by a pyrazine bridge and the extra electron is delocalized across both.
In the Creutz-Taube ion, a pyrazine ligand bridges two ruthenium centres and delocalizes the shared electron across both, a frozen snapshot of the bridge that fleetingly carries electrons in inner-sphere reactions.
A bridging ligand that conducts electrons can hold a shared electron between two metals indefinitely, not just during a fleeting transfer.
The bridging ligand is not a passive spacer: how strongly inner-sphere rates depend on which bridge is used is the best evidence the reaction is inner-sphere at all. Whether the bridge is truly reduced (chemical mechanism) or merely conducts (resonance mechanism) is a subtle, sometimes still-debated distinction.