The kinetics of the electron transfer reactions between complexes have been studied and occur by outer sphere and inner sphere mechanisms. (see examples). In the first mechanism the transfer of electrons occurs through the outer electron shell of the complex. In the second through an intermediate with a bridging ligand.
When these changes in oxidation states metals affect the occupation of the orbitals eg, of the (t2g)n (eg)m configuration, the electron transfer is slow. Otherwise, it can be fast. In the inner-sphere mechanism generally, the greater the difference between the standard potentials of both systems, the faster is the reaction.
The complex diffuse to join in a "box" within the solvent, producing the electron transfer through its outer electronic shells. The products separate leaving intact their coordination spheres. M-L distances can be modified slightly due to changes in the charge and in the central atom radii. This mechanism is called outer sphere .
When changes in oxidation states metals affect the occupation of the orbitals eg, the settings (t2g)n(eg)m, electron transfer will be slow. Otherwise, it can be fast.
The hexaacuochrome (II) is labile and Cl can act as a bridge, so it forms the binuclear complex. Electron transfer is achieved through the bridge Co-Cl-Cr and then takes place substitution of Cl by H2O in the coordination sphere of Co. The reaction in acid medium is followed by the transformation of the ligands NH3NH4+ and coordination of water molecules cobalt. The result is:
[CoCl(NH3)5]2+ + [Cr(H2O)6]2+ + 5[H3O)]+ ⇄ [Co(H2O)]62+ + [CrCl(H2O)5]2+ + 5[NH4)]+