Consecutive ligand-based PCET processes affording a doubly reduced nickel pyrazinedithiolate which transforms into a metal hydride required to evolve H 2

Our DFT results demonstrate that hydrogen evolution from water catalyzed by a nickel pyradinedithiolate (dcpdt) molecular hydrogen evolution catalyst [NiII(dcpdt)2]2- proceeds via the formation of a square-planar nickel(ii) hydride intermediate which is given by unprecedented structural transformation of a doubly reduced triply protonated species [NiII(dcpdtH2)(dcpdtH)]-, afforded as a result of two consecutive ligand-based reductions of [NiII(dcpdt)(dcpdtH)]- through proton-coupled electron transfer (PCET) pathways.