Outer-Sphere Transition States

This problem has been addressed recently197 with model reactions in which the nucleophile (viz. electron donor) is NO-. It has been found that outer-sphere transition states are easier to compute with NO- than with other electron donors, thus allowing a complete characterization of the competition between the various pathways. 

Gram-positive outer-sphere transition-state... 

This approach to mechanism diagnosis is much less straightforward with uncharged reactants, although in principle a distinction between inner- and outer-sphere transition states can still be achieved if high adsorbate coverages are employed. 

Three factors are relevant for the formation of the outer-sphere transition state the reagents must approach each other, the electronic interaction must be large enough —or a nonadiabatic tunneling mechanism needs to be operative—and the restriction imposed by the lack of nuclear motion during electronic motion must be satisfied. The energy of the transition state is determined by these three factors. 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

Thirdly, it will be important to gain more direct information on the stability of outer-sphere precursor states, especially with regard to the limitations of simple electrostatic models (Sect. 4.2). One possible approach is to evaluate Kp for stable reactants by means of differential capacitance and/or surface tension measurements. Little double-layer compositional data have been obtained so far for species, such as multicharged transition-metal complexes, organometallics, and simple aromatic molecules that act as outer-sphere reactants. The development of theoretical double-layer models that account for solvation differences in the bulk and interfacial environments would also be of importance in this regard. 

The Marcus treatment applies to both inorganic and organic reactions, and has been particularly useful for ET reactions between metal complexes that adopt the outer-sphere mechanism. Because the coordination spheres of both participants remain intact in the transition state and products, the assumptions of the model are most often satisfied. To illustrate the treatment we shall consider a family of reactions involving partners with known EE rate constants. 

Here we mention as an example that in the coordination-chemistry field optical MMCT transitions between weakly coupled species are usually evaluated using the Hush theory [10,11]. The energy of the MMCT transition is given by = AE + x- Here AE is the difference between the potentials of both redox couples involved in the CT process. The reorganizational energy x is the sum of inner-sphere and outer-sphere contributions. The former depends on structural changes after the MMCT excitation transition, the latter depends on solvent polarity and the distance between the redox centres. However, similar approaches are also known in the solid state field since long [12]. 

Fig. 5. A model for S042- substitution on [Be(H20)4]2+ proceeding from the outer-sphere complex on the left through the transition state at center to the inner-sphere complex at the right of the figure (16, 66).

The simplest electron transfer reactions are outer sphere. The Franck-Condon principle states that during an electronic transition, electronic motion is so rapid that the metal nuclei, the metal ligands, and solvent molecules do not have time to move. In a self-exchange example,... 

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