Metal phosphides have emerged as a new powerful class of materials that can be employed as heterogeneous catalysts in transformations mainly to generate new energy vectors and the valorization of renewables. Synthetic protocols based on wet techniques are available and are based on the decomposition of the organic layer decorating the nanoparticles. For nickel, the phosphine of choice is trioctylphosphine, and this leads to the formation of NiP_x materials. However, the temperature at which the decomposition starts has been found to depend on the quality of the nickel surface. Density functional theory, DFT, holds the key to analyze the initial steps of the formation of these phosphide materials. We have found how clean nickel surfaces, either (111) or (100), readily breaks the ligand P−C bonds. This triggers the process that leads to the replacement of a surface nickel atom by P and concomintantly forms a Ni adatom on the surface surrounded by two methyl groups, thus starting the formation of the NiP_x phase. The whole process requires low energies, in agreement with the low temperature found in the experiments, 150 °C. In contrast, if the surface is oxidized, the reaction does not proceed at low temperatures and oxygen vacancies need to be created first to start the P–C bond breaking on the Ni-clean patches. Our results show that the cleaner the surface is, the milder the reactions are required for the NiP_x formation, and thus they pave the way for gentler synthetic protocols that can improve the control of these materials.