Computational studies were carried out on three Zn(II) and three Ni(II) complexes, namely [NiL1(phen)2], [ZnL1(phen)2], [NiL2(phen)2], [ZnL2(phen)2], [NiL3(phen)2] and [ZnL3(phen)2]. These complexes were synthesized in 2014 by Raman and Mahalakshmi. Optimized structures of the studied complexes were calculated. IR and UV–Vis spectra were calculated and examined in detail. Additionally, non-linear optical (NLO) properties and natural electron configurations of the metal atoms were investigated using some quantum chemical parameters and natural bond orbital (NBO) analysis.