The synthesis, characterization, nonlinear optical properties and DFT calculations for a new Ni-diiminedithiolate complex [Ni(Cl-4-phen)(mi-5edt)] (Cl-4-phen = 3,4,7,8-tetrachloro-1,10-phenanthroline; mi-5edt = 1-(N-methylindol-5-yl)-ethene-1,2-dithiolate), are reported. The UV-Vis spectrum shows a solvatochromic absorption at 700 nm in DMF (epsilon = 5520 mol(-1) dm(3) cm(-1)) due to a charge-transfer (CT) highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transition, typical of push-pull complexes. The CT character of this electronic transition is confirmed by DFT calculations with the HOMO mainly centered on the mi-5edt moiety and the LUMO on the Cl-4-phen ligand. By comparison with the unsubstituted analogue compound, computational studies confirm the role of the chlorination enhancing the optical properties of this complex. The second order nonlinear optical properties were measured by EFISH technique (at 1.907 mu m), giving values of -2030 and -810 x 10(-48) esu for mu beta(1.907) and mu beta(0), respectively. These values are among the highest reported so far for the class of d(8) square-planar push-pull compounds.