Syntheses and Properties of Dinuclear Group 6 Metal Complexes with the Zwitterionic Sulfur Donor Ligand Bis(<i>N</i>,<i>N</i>‑diethylamino)carbeniumdithiocarboxylate

A class of dinuclear group 6 metal complexes [{M⁰(CO)₅}₂(μ-<i>S</i>,<i>S′</i>-EtL)] (M = Cr, Mo, W) with the asymmetrically bridged zwitterionic sulfur donor bis­(<i>N,N</i>-diethylamino)­carbeniumdithiocarboxylate (EtL) was prepared by photoreaction of [M⁰(CO)₆] with EtL in hexane–CH₂Cl₂ for Cr and Mo or THF for W. The same structure in a <i>syn</i>–<i>anti</i> coordination mode was revealed by X-ray analyses for chromium(0) <i>and</i> tungsten(0) complexes, <b>1</b> and <b>3</b>, respectively, and speculated from various analytical data for the molybdenum(0) complex <b>2</b>. The pertinent criteria for the coordination mode are found to be the wavenumber of the asymmetric stretching vibration of the −CS₂ moiety in the solid state and the ligand-based reduction potential in solution. Complexes <b>1</b> and <b>3</b> showed quasi-reversible reduction waves, which are assigned to a two-step, one-electron reduction process derived from the bridging EtL. In a series of [Cr­(CO)₅L] complexes (L = monodentate ligand), the ¹³C NMR chemical shifts of the <i>trans</i>-to-L carbonyl group showed the order of increasing σ-donor/π-acceptor ratio of the ligands. The sulfur donor ligands, including EtL and thione, are positioned in the middle of this range. All complexes exhibited negative solvatochromism: the wavelength of the absorption maximum shifted to the blue side in the range 500–600 nm with an increase in the Reichardt solvent polarity parameters <i>E</i>_T(30), except for protic solvents. Quantum chemical calculations by time-dependent density functional theory–polarized continuum model were employed for understanding the excited states and solvatochromic properties of complex <b>3</b>. The calculated vertical excitation energies in solution are consistent with the experimental data, suggesting that the transition is a metal-to-ligand charge-transfer transition. In addition, UV–vis, NMR, and cyclic voltammetry data showed that complex <b>3</b> dissociates into two mononuclear species in polar solvents: [W­(CO)₅(EtL)] and [W­(CO)₅(solvent)]