Equations for the activation free energy Delta F-# and preexponential factor of the rate constant of the redox reaction X-2 + e--> X-2(-) in solution characterized by a considerable elongation of the chemical bond X-X are obtained. Intramolecular potential energies describing the stretching vibrations along the X-X bond in the molecule and its anion are approximated by Morse functions. The dependencies of Delta F-# and other characteristics of the transition configuration on the free energy of the reaction are compared with those obtained in harmonic approximation.