This work presents a combined experimental and theoretical study on new synthesized ONN and ONS chelating 2-hydroxyacetophenone thiosemicarbazone ligands and their nickel(II) complexes with triphenylphosphine co-ligand. The 5-bromo-2-hydroxyacetophenone thiosemicarbazone ligand (L-1) coordinates to nickel through the phenolic-O, azomethine-N and thiole-S atoms and the complex [Ni(L-1) (PPh3)] (1) is formed an ONSP donor set with P atom of triphenylphosphine ligand. The 5-bromo-2-hydroxyacetophenone-S-methyl-thiosemicarbazone ligand (L-2) is functional a thioamide nitrogen instead of a sulfur atom, so complex [Ni(L-2)(PPh3)] (2) have ONNP donor set. The spectroscopic properties of all compounds have been determined by IR, H-1 NMR and UV-Vis spectroscopy techniques and the crystal structure of L-1, 1 and 2 have also been studied using X-ray diffraction. The molecular geometries obtained by X-ray analyzes in the ground state were compared with the optimized geometries which were calculated using the DFT/B3LYP method. The 6-311G(d,p) basis set for C, H, N, O, P, S atoms and LANL2DZ basis set for Ni atom were chosen in all theoretical calculations. In addition to molecular geometries, the vibrational frequencies, electronic transitions and H-1 NMR chemical shifts of the compounds were computed and compared with the experimental values. The electronic absorption spectra of the both complexes were predicted by using the time-dependent DET method. The HOMO-LUMO analyses were carried out and the chemical reactivity parameters (chemical hardness and softness, electronegativity, chemical potential and electrophilicity index) were calculated for studied compounds. (C) 2017 Elsevier Ltd. All rights reserved.