Thiolato-bridged cyclopentadienylnickel dimeric complexes have been prepared and found to be efficient and robust proton reduction electrocatalysts using acetic acid as the proton source. From cyclic voltammetry studies, moderate overpotentials of around 0.6 V and i c / i p values from 7.8 to 12.2 have been determined for 20 equiv of acetic acid at a scan rate of 100 mV/s. A turnover number of around 7 has been determined for each of the nickel complexes. The thiolato substituent of the complex does not appear to influence the catalysis significantly. Each of the nickel complexes acts as a robust homogeneous catalyst that could sustain continuous proton reduction for hours. On the basis of the experimental data, an electrochemical− chemical−electrochemical−chemical mechanism describing the catalytic process has been proposed as well.