On the thus formed low valent cobalt catalytic active species H2O-Co 2+ -N(pyr), DFT calculations predict a spontaneous adsorption of acetylene (ΔG = -0.63 eV). Coupled proton and electron transfer then occurs from the adsorbed water and the metal center, respectively, to the adsorbed acetylene as a result of water dissociation although with a free energy penalty of 0.55 eV. Our result also aligns with our previous work on the homogeneous porphyrin-based system for acetylene semihydrogenation , as well as with other studies on single-atom catalysts for semi-hydrogenation reactions, where water dissociation and protonation of the π-complex are the rate determining step. A second photoexcitation and transfer of a second proton was simulated by adding a photoexcited electron to Co 2+ and introducing another water molecule. A similar coupled proton and electron transfer from the water and Co to the adsorbed *H-C=CH2 is predicted (FigureS18 in the Supplementary Information). Finally, calculations predict high thermodynamic energy cost for further hydrogenation of the C=C double bond (Figure in the Supplementary Information). In comparison, the desorption of ethylene is favored, closing the catalytic cycle and ensuring highly selective semi-hydrogenation (Figure , Supplementary Information).