Rational catalyst design is an important strategy for improving the efficiency of conversion of low-energy feedstocks like CO 2 and H 2 O into renewable chemical fuels. Hydrogen (H 2 ) generation receives significant attention in this area, as the reaction involves transfer of multiple protons (H + ) and electrons (e -). Effective catalysts for H 2 generation avoid high-energy barriers while also coupling the individual H + and e -transfers involved in the catalytic cycle together to drive H -H bond formation. However, obtaining information on the elementary reaction steps involved in catalysis is challenging, because common methods like electrochemistry can drive catalysis but offer little structural insight into the reactivity happening under catalytic conditions. In light of the relevance of catalyst design to myriad issues in sustainability science, significant experimental and computational work has focused on identifying the key intermediates involved in hydrogen evolution with molecular catalysts. One notable catalyst for hydrogen generation, originally reported by Grätzel and Kölle in 1987, is built on the [Cp * Rh] motif, where Cp * = η 5 -pentamethylcyclopentadienyl. Catalysts in this family are supported by additional bidentate diimine ligands, such as 2,2'-bipyridyl (bpy) or 1,10-phenanthroline (phen), and are effective for hydrogen generation under both electrochemical and photochemical conditions. [Cp * Rh] complexes have also found utility in catalytic conversion of NAD(P)+ to NAD(P)H under bioelectrocatalytic conditions. Synthetic manipulation and handling of [Cp * Rh] is straightforward and modular, facilitating derivatization efforts, incorporation into more elaborate constructs and devices, and detailed mechanistic work. Interest in the [Cp * Rh] catalysts was recently refreshed by reports showing the formation of the η 4 -pentamethylcyclopentadiene (Cp * H) motif upon reduction and protonation of reduced species that can be generated from precursors such as [Cp * Rh(bpy)Cl] + . Specifically, protonation of Cp * Rh(bpy) or Cp * Rh(phen) with 1 equiv. of acid results in formation of new rhodium(I) species bearing η 4 -Cp * H that can be isolated in pure form. However, these new species bear exclusively endoη -Cp * H, with the special ring proton oriented toward the rhodium(I) center. These findings, along with very recent time-resolved spectroscopic studies that have identified the transient hydride supported by bpy, [Cp * Rh(bpy)H] + and allowed for its kinetic characterization, strongly support that transient [Cp * Rh] hydride complexes are involved in this chemistry. These hydrides form by protonation of the reduced forms and undergo proton-hydride tautomerization that results in transfer of the proton to the Cp * ring. The involvement of transient hydride complexes in the catalytic chemistry mirrors the original mechanistic proposals for hydrogen generation by this catalyst; following reduction to the Rh(I) oxidation state, the first protonation was anticipated to generate a Rh(III)-hydride which could undergo protonolysis by a second H + equivalent to generate H 2 . Summarizing the consensus catalytic cycle 4,13 for these [Cp * Rh] complexes that is relevant to work in strong acid, four key species are believed to be involved in the catalytic cycle and these are shown in Figure . The main cycle begins with a Rh(I) species, which is protonated to generate the hydride, Rh(III)H (Reaction 1).