A series of [M(diimine)(dithiolate)] complexes of general formula [M(bpy)(dithiolate)] {bpy=2,2′-bipyridine;dithiolate=1,2-benzenedithiolate (bdt2−), 3,4-toluenedithiolate (tdt2−) and 4-cyanobenzene-1,2-dithiolate (cbdt2−); M=Ni(II), Pd(II) and Pt(II)} have been studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The geometries, stabilities, electronic structures, optical absorption spectra in different phases as well as thermodynamic parameters are explored. The changes of metal ion center and dithiolate ligands on some molecular properties are also discussed. These calculated results are in good agreement with the experimental data. The bonding analyses show that the MS bond is covalent so that always polarized towards sulfur atom, whereas the MN bond exhibits a considerable amount of electrostatic interaction. Detailed NBO analysis indicates that these complexes can be easily oxidized than reduced, and acts as the reducing agent. The HOMO-LUMO energy gaps of all complexes under study are founded about 2eV and the strong absorption from 400 to 700nm which match with the solar spectra very well. Besides, the simulated absorption spectra are in accordance with the trends of energy gaps. Comparison of the absorption spectra in dichloromethane solution with those in gas phase show that the solvatochromic effect. The order of magnitude for light harvesting efficiencies (LHE) of all complexes is Pt>Pd>Ni and cbdt2−>bdt2−>tdt2−. Our results confirm the effect and role of metals and dithiolate ligands on enhancing the optical properties of these complexes. Thus, the result of this work can serve as a rational tool for the design and synthesis of diimine-dithiolate complexes and broadens the scope for further investigations into potential dyes for use in the field of dye-sensitized solar cells (DSSC).