Marcus Theory - Outer Sphere Electron Transfer

Outer Sphere Electron Transfer

Redox reactions are preferably run in polar solvents. Donor and acceptor then have a solvent shell and the precursor and successor complexes are solvated also. The closest molecules of the solvent shell, or the ligands in complexes, are tightly bound and constitute the "inner sphere". Reactions in which these participate are called inner sphere redox reactions. The free solvent molecules constitute the "outer sphere". Outer sphere redox reactions do not change the inner sphere, no bonds are made nor broken.

It was R.A. Marcus who realized the role of the solvent when he worked on the nature and magnitude of the Gibbs free energy of activation for redox reactions, more precisely: one-electron transfer reactions of the outer sphere type. He published two fundamental papers. The ideas of these two papers are often referred to Marcus Theory although Marcus’ later work goes much beyond them. In the following the development and results of the ideas of these two papers are outlined. For the mathematics and more details the original papers should be consulted.