Modification of transition metals with p-block elements is known to be effective to tune the ensemble characteristics of catalysts for the semi-hydrogenation of alkynes. To further explore this approach, here we prepare two nickel phosphides, namely Ni2P and Ni5P4. Assessment in the semi-hydrogenation of 1-hexyne and 2-methyl-3-butyn-2-ol shows that the phosphides present higher rate and selectivity than unmodified nickel catalysts. While no activity and selectivity differences are displayed in the semi-hydrogenation of 1-hexyne over Ni2P and Ni5P4, in the case of 2-methyl-3-butyn-2-ol a higher rate and lower selectivity to 2-methyl-3-buten-2-ol are observed over Ni2P. Density functional theory reveals that the hydroxyl group facilitates the reaction, but also increases the barrier for product desorption. Detailed analyses of the ensemble show the potential of phosphorus to create spatially-isolated nickel trimers that surpass the performance of unmodified nickel, but also its limited ability to modulate the electronic properties and related binding energies of organic intermediates, which is key to preventing undesired side reactions.