AbstractMetal carbonyl clusters (MCC's) are atomically defined nanomaterials which can be characterized using the precise tools of molecular (electro)chemistry. HER mechanisms involve two proton transfer (PT) steps in the catalytic cycle. For HER catalyzed by [Co11C2(CO)23]3− (13−), cyclic voltammetry measurements were used to determine the rate for PT1 as kPT1=3×108 M−1 s−1, whereas the rate for PT2 is kPT2=3.7×103 M−1 s−1. The fast, diffusion‐limited rate for PT1 is consistent with a previous report describing [Co13C2(CO)24]4− (24−), with PT=2.9×109 M−1 s−1. In both cases rate enhancement in PT1 is promoted by the many Co−Co bonds on the surface of the MCC that serve as PT sites: a statistical enhancement of rate akin to the effects of proton relays. In contrast, kPT2 varies and is five orders of magnitude slower for 12− compared with 24−. Thus, MCC's and nanomaterials offer an opportunity to enhance the rate for PT1 while maintaining thermochemical or kinetic control of PT2.