We report the formation of nickel borides, at room temperature and pressure, from the decomposition of NaBH4 promoted by the addition of nickel bromide at different concentrations in a dispersing organic medium, tetrahydrofuran and pentane. The nickel borides, formed as amorphous powders, were analyzed, and the structure information served as input for modeling a periodic lattice structure with the same composition. Experimentally, the nickel boride phases were predominantly composed of a boron-rich phase with composition NiB3. Combining FT-IR, X-ray diffraction analyses, and theoretical structure determination, we suggest for it a monoclinic structure, with symmetry group P2(1)/c(1) lattice parameters a = 3.038 angstrom, b = 8.220 angstrom, c = 5.212 angstrom, alpha = beta = 90.00 degrees and gamma = 87.57 degrees. The enthalpies of formation of the nickel boride phases, as well as the lattice stability, were calculated using density functional theory and density functional perturbation theory methods.