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 [Ni ^II (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 [Ni ^II (dcpdtH ₂ )(dcpdtH)] ⁻ , afforded as a result of two consecutive ligand-based reductions of [Ni ^II (dcpdt)(dcpdtH)] ⁻ through proton-coupled electron transfer (PCET) pathways.