Proton abstraction leading to the formation of a hydride species required to evolve H2 largely relies on the basicity of d orbital of the metal responsible for this action. Here we report that a square-planar NiII(bpy)(dcbdt) hydrogen evolution catalyst shows substantial acceleration in the proton abstraction rate due to the increased basicity at the filled Ni dz2 orbital after formation of [NiI(bpy−.)(dcbdt)]2− via consecutive two one-electron reductions (bpy=2,2′-bipyridine; dcbdt=4,5-dicyanobenzene-1,2-dithiolate). The catalyst is likely to adopt the EECC′ mechanism in which the rate of the first protonation step is by far higher than that of the second step, even though an alternative path requiring another reduction (i. e., ECEC′) remains unexcluded. Our DFT calculations reveal that the first and second reductions are correlated with the electron injection into the metal-ligand anti-bonding and π*(bpy) orbitals, respectively, where the latter orbital shows non-negligible hybridization with the nickel d orbital. In addition, a homoleptic catalyst [NiII(dcbdt)2]2− is shown to adopt the EC′EC mechanism with the rate-determing step being a hydride forming step, consistent with the largely delocalized nature of the injected electron over the two dcbdt ligands (π*(dcbdt) orbital). This work demonstrates the importance of raising the basicity of the metal d orbital, relevant to promote the proton-coupled electron transfer (PCET).
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