Electronic Structure of Square Planar Bis(benzene-1,2-dithiolato)metal Complexes [M(L)2]z (z = 2-, 1-, 0; M = Ni, Pd, Pt, Cu, Au): An Experimental, Density Functional, and Correlated ab Initio Study

The three diamagnetic square planar complexes of nickel(II), palladium(II), and platinum(II) containing two S,S-coordinated 3,5-di-tert-butylbenzene-1,2-dithiolate ligands, (LBu)2-, namely [MII(LBu)2]2-, have been synthesized. The corresponding paramagnetic monoanions [MII(LBu)(LBu(bullet))]- (S = 1/2) and the neutral diamagnetic species [MII(LBu(bullet))2] (M = Ni, Pd, Pt) have also been generated in solution or in the solid state as [N(n-Bu)4][MII(LBu)(LBu(bullet))] salts. The corresponding complex [CuIII(LBu)2]- has also been investigated. The complexes have been studied by UV-vis, IR, and EPR spectroscopy and by X-ray crystallography; their electro- and magnetochemistry is reported. The electron-transfer series [M(LBu)2]2-,-,0 is shown to be ligand based involving formally one (LBu(bullet))- (pi)radical in the monoanion or two in the neutral species [MII(LBu(bullet))2] (M = Ni, Pd, Pt). Geometry optimizations using all-electron density functional theory with scalar relativistic corrections at the second-order Douglas-Kroll-Hess (DKH2) and zeroth-order regular approximation (ZORA) levels result in excellent agreement with the experimentally determined structures and electronic spectra. For the three neutral species a detailed analysis of the orbital structures reveals that the species may best be described as containing two strongly antiferromagnetically interacting ligand radicals. Furthermore, multiconfigurational ab initio calculations using the spectroscopy oriented configuration interaction (SORCI) approach including the ZORA correction were carried out. The calculations predict the position of the intervalence charge-transfer band well. Chemical trends in the diradical characters deduced from the multiconfigurational singlet ground-state wave function along a series of metals and ligands were discussed.