inner-sphere electron transfer

Go back to the two people passing a coin, but this time they reach out and grip opposite ends of the same umbrella before the coin slides along the handle from one to the other. They are physically connected during the hand-off. Inner-sphere electron transfer works like that: the two metal complexes share a single ligand that bridges between them, and the electron travels through that shared bridge.

In the inner-sphere mechanism the two metals do not stay aloof. One ligand — typically a halide, hydroxide, or another group with a lone pair to spare — leaves the coordination shell of one complex and bonds simultaneously to both metals, forming a bridged binuclear species in which the bridging ligand links donor and acceptor. The electron then passes from one metal to the other along this bridge, and afterward the bridge often breaks so that the bridging ligand ends up carried away with one of the metals. Because forming the bridge requires a ligand to enter or leave a coordination sphere, at least one of the partners must be kinetically labile enough to open a site for the bridge. This is the crucial structural and kinetic difference from the outer-sphere route, where coordination shells stay intact.

Henry Taube proved this mechanism exists with one of the most beautiful experiments in inorganic chemistry, work that won him the 1983 Nobel Prize. He reacted inert [Co(NH3)5Cl]2+ (cobalt(III), with a chloride ligand) with labile [Cr(H2O)6]2+ (chromium(II)). After electron transfer the product was [Cr(H2O)5Cl]2+ — the chloride had migrated from cobalt to chromium. Since the resulting chromium(III) is itself inert and could not have picked up chloride after the fact, the chloride must have been the bridge through which the electron passed, caught red-handed riding across as the metals exchanged the electron. It is a rare case where a fleeting reaction mechanism leaves an unmistakable, isolable fingerprint.