Vibrational Markers for the Open-Shell Character of Transition Metal Bis-dithiolenes: An Infrared, Resonance Raman, and Quantum Chemical Study

Transition metal complexes involving the benzene-1,2-dithiol (L2-) and Sellmann's 3,5-di-tert-butylbenzene-1,2-dithiol(LBu 2-) ligands have been studied by UV−vis, infrared (IR), and resonance Raman (rR) spectroscopies. Raman spectra were obtained in resonance with the intervalence charge transfer (IVCT) bands in the near-infrared region and ligand-to-metal charge transfer (LMCT) bands in the near-UV region. Geometry optimization and frequency calculations using density functional theory (DFT) have been performed for [M(L)2]z and [M(LBu)2]z species (M = Ni, Pd, Pt, Co, Cu, Au, z = −1; M = Au, z = 0). On the basis of frequency calculations and normal-mode analysis, we have assigned the most important totally symmetric vibrations as well as corresponding overtone and combination bands that appear in rR spectra of compounds [Ni(L)2]1-, [M(LBu)2]1- (M = Ni, Pt, Co, Cu). Experimental values of dimensionless normal coordinate displacements in excited states have been determined by fitting of rR spectra together with the absorption band shape, based on the time-dependent theory of Heller. Time-dependent density functional theory (TD-DFT) and multireference post-Hartree−Fock ab initio calculations, using the difference dedicated configuration interaction (MR-DDCI) method, were carried out to evaluate dimensionless normal coordinate displacements quantum chemically. The calculations show encouraging agreement with the experimental values. The large distortions along several normal modes led to significant vibronic broadening of IVCT and LMCT bands, and the broadening was accounted for in the deconvolution of the absorption spectra. The presence of an intense rR band around ∼1100 cm-1 was found to be a reliable marker for the presence of sulfur-based radicals.