Metal-organic frameworks (MOFs) are excellent candidates for achieving this goal through a molecular approach to heterogeneous catalysis. MOFs are a class of porous, crystalline, multidimensional nanomaterials composed of inorganic metal ions or clusters (nodes) connected by polytopic organic molecules (linkers). Pursuing the use of MOFs in catalysis can offer some additional advantages, such as: (i) their chemical stability and high surface areas make them great supports for highly dispersed and well-isolated catalysts; (ii) the plethora of choices for both the inorganic and organic components of MOFs, allowing for the incorporation of molecular catalysts into the framework as either the node or the linker; (iii) MOFs are also prized for the ease with which they can be post-synthetically modified, which allows catalysts to be installed and distributed throughout the framework after synthesis; (iv) the crystallinity and long-range order of MOFs make it is possible, in principle, to engineer the same microenvironment for all catalyst species to achieve true single-site catalysis. In the previously reported photocatalytic reaction conditions for acetylene semi-hydrogenation with cobalt porphyrins, the hydrogenation was proposed to proceed through the acetylene πcomplex pathway, which could limit the activity due to the protonation of the π-complex being the rate determining step of the catalytic cycle. An alternative route to alkyne hydrogenation could be a hydrogen atom transfer (HAT) reaction, through a metal hydride intermediate. We therefore hypothesized that catalysts capable of performing HAT reactions via a Co III -H intermediate would be good candidates for photocatalytic acetylene hydrogenation. Therefore, to improve upon Co-porphyrin-based systems, and to emphasize the ability of MOFs to stabilize delicate homogeneous catalysts, here we pursue cobaloxime-based systems for the photocatalytic semi-hydrogenation of acetylene. Cobaloximes are octahedral Co complexes composed of two equatorial dioxime ligands, commonly connected by hydrogen bonding (Figure ). The upward axial ligand is typically a halide, while the downward axial ligand can be either another halide, or a nitrogen base such as pyridine. The relative ease of preparation, together with synthetic flexibility and tunability of the ligand structure, have made cobaloximes a popular choice for development of photocatalytic systems. Cobaloximes are among the most widely studied molecular catalysts for hydrogen evolution, exhibit excellent activity, and can be paired with numerous organic and inorganic photosensitizers. Furthermore, it is well-established that cobaloxime-mediated H2 evolution proceeds through a Co III -H intermediate. Cobalt hydride intermediates have been also reported for HAT reaction to organic substrates, which suggests that this method could be extended to acetylene semi-hydrogenation. As they exist in the same phase as the reaction mixture, separation and reuse of molecular cobaloximes from the reaction mixture can be, however, extremely difficult or impossible. Furthermore, molecular cobaloximes are known for their long-term instability due to only hydrogen bonds holding together the equatorial ligand framework. To stabilize the cobaloxime motifs, the following strategies can be pursued: (i) excess ligand is often added to stabilize the cobaloximes in catalytic studies; (ii) the internal hydrogen bonding of the oximes can be replaced by difluoroborylate (BF2) groups; and (iii) cobaloximes can be stabilized by incorporation into a MOF. Here, we report two novel photocatalytic systems for the semi-hydrogenation of acetylene to ethylene: one homogeneous system using a molecular cobaloxime and one heterogeneous system based on a metal-organic framework with a cobaloxime linker (Figure ). The catalysts are photosensitized by tris(2,2'-bipyridyl)ruthenium(II) chloride ([Ru(bpy) dimethylglyoxime, py = pyridine), which exhibits excellent catalytic activity for the conversion of acetylene to ethylene with turnover number (TON) = 1,148 and >99.9% selectivity for ethylene vs. ethane after 20 hours of visible-light irradiation (450 nm). The homogeneous system is also highly competent for ethylene purification (industrially relevant conditions), achieving nearly 100% visible-light powered conversion of acetylene to ethylene in just 50 minutes, with no evidence of over-hydrogenation to ethane, such that the selectivity for ethylene vs. ethane is >99.9%. Detailed mechanistic studies reveal that acetylene semi-hydrogenation relies on production of a Co I active species that is protonated to form a Co III -H intermediate, as anticipated.