The Marcus theory offers a comprehensive explanation of electron transfer reactions, distinguishing between inner-sphere and outer-sphere mechanisms.

Inner-Sphere Mechanisms
Inner-sphere mechanisms involve direct interactions between the donor and acceptor molecules, where an electron is transferred through a coordinated bond. This type of mechanism is more likely to occur when the donor and acceptor are in close proximity and possess compatible coordination geometries. The coordinated bond can form through ligands or solvent molecules, leading to changes in the oxidation states of both the donor and acceptor.

Outer-Sphere Mechanisms
Conversely, outer-sphere mechanisms describe the transfer of an electron via a surrounding solvent or medium, without direct contact between the donor and acceptor molecules. This mechanism is favored when the donor and acceptor are situated farther apart and exhibit incompatible coordination geometries. In this case, the electron transfer can alter the electrostatic potential of the surrounding solvent or medium, which may influence the reaction rate.

In summary, the Marcus theory provides a crucial framework for understanding electron transfer reactions and the various factors that affect their rates. By analyzing both inner-sphere and outer-sphere mechanisms, researchers can more effectively predict and manipulate these reactions in a wide range of applications, including energy conversion and storage.