Experimental estimate of the electron-tunneling distance for some outer-sphere electrochemical reactions

Estimates of the reaction zone thickness over which electron tunneling can effectively occur for the outer-sphere electrochemical reduction of some Cr(III) complexes are obtained by comparing the observed work-corrected rate constants with unimolecular rate constants for the electroreduction of structurally similar surface-bound Cr(III) reactants. Effective reaction zone thicknesses of ca. 0.1-0.3 and ca. 5 Å are obtained for outer-sphere electron transfer with Cr(III) reactants containing predominantly aquo or ammine ligands, respectively. This indicates that the former reactions are marginally nonadiabatic whereas the latter are decidedly adiabatic at their respective planes of closest approach. These findings are compatible with the greater reactant-electrode separation distances previously noted for Cr(III) aquo relative to ammine complexes resulting from the more extensive hydration sheath surrounding the former reactants. Comparisons with recent calculates performed for some outer-sphere homogeneous reactions suggest that efficient electron tunneling takes place over roughly comparable distances at metal-electrolyte interfaces and in bulk solution.