Electron transfer rates in Marcus Theory
There is a description of Marcus theory here How does the inverted Marcus region explain chemiluminescence? . The quantum nature can be added into the theory as necessary, it leads to a asymmetrical plot of rate constant vs free energy with the inverted region (large -$\Delta G$) decaying away more slowly than the normal region.  The equilibrium is between the solvent reorganisation with reactants and then with products. The solvent plays a controlling role in the process, the proof is that experiment demonstrates that  the rate of electron transfer can be limited by solvent motion. See Harrison et al.  Chemical Physics, Volume 116, Issue 3, 15 September 1987, Pages 429-448, and Irvine et al. ibid.  Volume 104, Issue 2, 1 May 1986, Pages 315-324. The novel aspect of Marcus theory is the role of the  solvent and this was not generally accepted until the inverted region was observed.