Synthesis, characterization, NLO properties, and theoretical studies of the mixed-ligand dithiolene complexes of the nickel triad [M(II)(Bz(2)pipdt)(mnt)] (Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione, mnt = maleonitriledithiolato, M(II) = Ni, 1, Pd, 2, Pt, 3) are reported. Molecular structural characterization of 1-3 points out that four sulfur atoms are in a slightly distorted square-planar geometry. While the M-S bond distances are only slightly different, comparison of the C-C and C-S bonds in the C2S2MS2C2 core allows us to point out a significant difference between the C-C and the C-S distances in Bz(2)pipdt and mnt. These findings suggest assigning a dithiolato character to mnt (pull ligand) and a dithione one (push ligand) to Bz(2)pipdt. Cyclic voltammetry of 1-3 exhibits two reversible reduction waves and a broad irreversible oxidation wave. These complexes are characterized in the visible region by a peak of moderately strong intensity, which undergoes negative solvatochromism. The molecular quadratic optical nonlinearities were determined by the EFISH tedmique, which provided the following values mu beta(lambda) (10(-48) esu) = -1436 (1), -1450 (2), and -1950 (3) converted in mu beta(0) (10(-48) esu) = -463 (1), -684 (2), and -822 (3), showing that these complexes exhibit large negative second-order polarizabilities whose values depend on the metal, being highest for the Pt compound. DFT and TD-DFT calculations on 1-3 allow us to correlate geometries and electronic structures. Moreover, the first molecular hyperpolarizabilities have been calculated, and the results obtained support that the most appealing candidate as a second-order NLO chromophore is the platinum compound. This is due to (i) the most extensive mixture of the dithione/metal/dithiolato orbitals, (ii) the influence of the electric field of the solvent on the frontier orbitals that maximizes the difference in dipole moments between the excited and the ground state, and (iii) the largest oscillator strength in the platinum case vs nickel and palladium ones.