A bond-breaking electron transfer from a metal to a diatomic molecule is considered, with focus on the effect of the reactant's separation from the electrode. When the reactant interacts only with a wide sp-band on the metal, the energy of activation is reduced with decreasing separation, but this effect is comparatively small and mainly caused by a decrease of the energy of reorganization. In contrast, a coupling to a narrow d-band centered near the Fermi level can lead to a drastic reduction of the activation energy, which is caused by a splitting of the molecule's antibonding orbital as the system passes the saddle point. (c) 2007 Elsevier B.V. All rights reserved.