Electronic and Molecular Structures of a Series of Nickel Bis-1,1-Dithiolates

A series of homoleptic Ni bis-1,1-dithiolates, [Ni(S₂C₂RR')₂]^2- (R=CN, R'=CN, CO₂Et, CONH₂, Ph, Ph-4-Cl, Ph-4-OMe, Ph-4-NO₂, Ph-3-CF₃, Ph-4-CF₃, Ph-4-CN; R=NO₂, R'=H; R=R'=CO₂Et) have been synthesized from the reaction of the alkali metal salt of the ligand and nickel chloride, and isolated as tetraphenylphosphonium or tetrabutylammonium salts. The complexes were characterized by X-ray crystallography, high-resolution mass spectrometry, and infrared (IR), nuclear magnetic resonance (NMR) and electronic absorption spectroscopies. The molecular structures show a rigidly square planar Ni(II) center linking two four-membered chelate rings whose dimensions are constant across the series. The electronic effect of the ligand substituent is revealed in the ¹³C NMR and electronic spectra, and corroborated by density functional calculations. Electron withdrawing groups deshield the low-field CS₂ resonance, and the signature charge transfer band in the visible region is red-shifted. These observables have been accurately reproduced computationally, and revealed the Ni contribution to the ground state diminishes with decreasing electron withdrawing capacity of the ligand substituent. In contrast to 1,2-dithiolates, the redox inactivity afforded by 1,1-dithiolates stems from the smaller chelate ring and substantially reduced sulfur content that is key to stabilizing the radical form.