Fe-2(bdt)(CO)(6) (1) (bdt = benzenedithiolate) exhibits a chemically reversible two-electron reduction at -1.27 V vs. Fc(+/0) in CH3CN + Bu4NPF6 electrolyte. The increase in height of the reduction wave of complex 1 upon increasing the concentration of toluenesulfonic acid (pK(a) similar to 8 in CH3CN) is consistent with a catalysis of the proton reduction reaction (k(cal) similar to 500 M-1 s(-1)). The reaction scheme, derived from the simulation of the voltammetric responses, involves the initial formation of the dianion 1(2-) followed by two successive reactions with acid. The last protonation step is the rate determining step in the catalytic cycle. Using experiments conducted with acetic acid (pK(a) similar to 23 in CH3CN), we further shown that the intermediate species 1H(-) is a significantly weaker base than the dianion 1(2-). (c) 2006 Elsevier B.V. All rights reserved.