The proton-reduction catalytic activity of two di-iron hydrogenase complexes, [(mu-S2C3H6)[Fe(CO)(3)] [Fe(CO)(2)(PPh3)] (1) and (mu-S2C3H6)[Fe(CO)(3)][Fe(CO)(2)(PPh2{(CH2)(2)SH})] (2), was investigated at polycrystalline and (111)-faceted Au electrodes in nonaqueous electrolyte. Compound (2) was irreversibly tethered to the surface through the SH group; (1) was present only in the unadsorbed (dissolved) state. No enhancement of the proton reduction reaction was observed with the homogeneous complex. Pronounced catalysis was exhibited by the heterogenized (surface-attached) material. Neither increase nor decrease in activity was observed when unadsorbed complex (2) was added to the solution of the heterogenized catalyst. The conclusion from these observations, that no catalysis transpires unless the subject molecular complex is tethered to the electrode surface, is totally unexpected; it runs counter to conventional wisdom that an untethered homogeneous electrocatalyst, especially one that requires a particular entatic (partially rotated) configuration to complete its function, would invariably perform better than its surface-immobilized counterpart. The heterogenized complex, present at rather low coverages due to its sizable adsorbed-molecule cross section, was further investigated by polarization-modulation Fourier transform infrared reflection absorption spectroscopy (PM-FT-IRRAS), high-resolution electron-energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The electrochemistry (EC) and STM results indicated that the catalytic activity of the immobilized complex is a function of its surface coverage but not of its spatial configuration; the catalytic sites are accessible regardless of the particular arrangement of the pendant active site with respect to the surface. The surface-immobilized complex suffered a non-negligible loss in catalytic activity after the ex situ experiments, perhaps due to (partial) decarbonylation. (C) 2014 Elsevier B.V. All rights reserved.