Role of the Acceptor in Tuning the Properties of Metal [M(II) = Ni, Pd, Pt] Dithiolato/Dithione (Donor/Acceptor) Second-Order Nonlinear Chromophores: Combined Experimental and Theoretical Studies

The mixed-ligand complexes [M­(II)­(Et₂dazdt)­(mnt)] (M = Ni, <b>1</b>; Pd, <b>2</b>; Pt, <b>3</b>) [Et₂dazdt = <i>N</i>,<i>N</i>′-diethyl-perhydrodiazepine-2,3-dithione; mnt = maleonitrile-2,3-dithiolate] have been prepared and fully characterized. X-ray diffractometric studies on <b>1</b>–<b>3</b> (the structure of <b>1</b> was already known) show that the crystals are isostructural (triclinic, <i>P</i>–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. <b>1</b>–<b>3</b> 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 <b>1</b>–<b>3</b> as well as on [Pt­(Bz₂pipdt)­(mnt)] (<b>4</b>) and [Pt­(Bz₂pipdt)­(dmit)] (<b>5</b>) 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 <b>5</b> can be attributed both to its lowest torsion angles and to the extent of the π system of its dithiolate ligand, dmit