Aminoguanidine (Amgu) nickel complexes containing nitrate and chloride with the formula [Ni(Amgu)₂]X₂ [X = NO₃^– (1) and Cl⁻ (2)] were prepared and characterized by analytical, Fourier-transform infrared (FTIR) spectroscopic and single crystal X-ray diffraction studies. Compound 1 crystallized in the triclinic crystal system of space group P-1 with Z = 1, and compound 2 crystallized in the monoclinic crystal system of space group P2₁/n with Z = 2. Both compounds have square planar geometry with two neutral aminoguanidine ligands acting as trans N,N-chelators. The N-N stretching of aminoguanidine ligands was corroborated by the FTIR spectroscopic peak observed at 1139 cm⁻¹. NiO nanoparticles were obtained by the decomposition of compounds 1 and 2, which were calcined at 400 °C for 4 h. The powder X-ray diffraction (PXRD) patterns confirmed that the prepared NiO nanoparticles had high purity and crystallinity. Therefore, these complexes may be used as solid-state precursors for the preparation of NiO nanoparticles owing to their low temperatures of decomposition.