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 C(2)S(2)MS(2)C(2) 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 technique, which provided the following values μβ(λ) (10(-48) esu) = -1436 (1), -1450 (2), and -1950 (3) converted in μβ(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.

The synthesis and characterization of Pd(phen)(bdt) (1) (phen = 1,10-phenanthroline, bdt = 1,2-benzenedithiolate) is presented. 1 crystallizes in the monoclinic space group P2(1)/c, alpha = 11.281(4) A, b = 20.498(8) A, c = 8.374(3) A, beta = 90.234(8), V = 1936.5(13) A(3), Z = 4, and is isostructural with its previously reported related complexes. The ground and low lying excited electronic states in 1 and in the related complexes Pd(bpy)(bdt) (2), Pt(bpy)(bdt) (3), Pt(bpy)(mnt) (4), and Pt(bpy)(edt) (5) [where bpy = 2,2'-bipyridine, edt = ethylene-1,2-dithiolate, and mnt = maleonitriledithiolate] are studied using density functional theory techniques. The electronic properties of 1-5 are studied using the B3LYP functional. Optimized geometries are compared to experimentally observed structures. Time dependent density functional theory (TDDFT) is employed to investigate the excited singlet and triplet states. The calculated energies of the lowest singlet state and the lowest triplet state in all five complexes are in considerable agreement with experimental data. It is shown that variation of both metal and dithiolate-ligand going from 1 and 2 to 3, 4, and 5 has a substantial impact on the spectroscopic and excited-state properties, indicating at the same time the mixed metal/dithiolate character of the HOMO orbital. All the low-lying transitions are categorized as MMLL'CT transitions. The emissive state of all complexes is assigned as a triplet dithiolate/metal to diimine charge transfer with differences in the structures of the emissions resulting from differences in the pi dithiolate orbital of the mnt, bdt, and edt as well as from differences in metal.

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The mixed-ligand complexes [M(II)(Et2dazdt)(mnt)] (M = Ni, 1; Pd, 2; Pt, 3) [Et2dazdt = N,N'-diethyl-perhydrodiazepine-2,3-dithione; mnt = maleonitrile-2,3-dithiolate] have been prepared and fully characterized. X-ray diffractometric studies on 1-3 (the structure of 1 was already known) show that the crystals are isostructural (triclinic, P-1), and two independent molecular entities are present in the unit cell. These entities differ in the orientation of the ethyl substituents with respect to the epta-atomic ring. In the C2S2MS2C2 dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the M-S bond distances involving the two ligands are similar, while the C-S bond distances in the C2S2 units exhibit a significant difference in Et2dazdt (dithione) and mnt (dithiolato) ligands. 1-3 show in the visible region one or two moderately strong absorption peaks, having ligand-to-ligand charge-transfer (CT) character with some contribution of the metal, and show negative solvatochromism and molecular quadratic optical nonlinearity, which was determined by the EFISH (electric-field-induced second-harmonic generation) technique. These complexes are redox active and show two reversible reduction waves and one irreversible oxidation wave. Theoretical calculations based on DFT and TD-DFT calculations on complexes 1-3 as well as on [Pt(Bz2pipdt)(mnt)] (4) and [Pt(Bz2pipdt)(dmit)] (5) highlight the factors which affect the optical properties of these second-order redox-active NLO chromophores. Actually, the torsion angle of the dithione system (δ2) inversely correlates either with the oscillator strengths of the main transition of the complexes or with their beta values. The high beta value of 5 can be attributed both to its lowest torsion angles and to the extent of the π system of its dithiolate ligand, dmit.

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Keywords: Density functional calculations / Fukui indices / Oxidation / S ligands / Schiff bases / Stacking interactionsFukui functions are widely used when investigating the reactivity of organic molecules, but rarely with metal complexes. Here, we investigate the reactivity of [(diimine)(dithiolato)M] complexes with different types of reagents and upon oxidation employing this concept. Mixed-ligand complexes of this type have a peculiar electronic description due to the mixed-metal-ligand-to-ligand charge-transfer band, which is why they are considered as very promising candidates for non-linear optical (NLO) materials and molecular photochemical devices (MPD). As a result, their reactivity is of crucial importance for their potential applications. The obtained

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