Electronic Delocalization, Energetics, and Optical Properties of Tripalladium Ditropylium Halides, [Pd3(C7H7)2X3]1− (X = Cl−, Br−, and I−)

Here we report relativistic electronic structure calculations employing all-electron density functional theory (DFT) including scalar and spin-orbit interaction, on the multimetallic sandwich compound [Pd(3)(C(7)H(7))(2)X(3)](1-) (X = Cl(-) (1), Br(-) (2), and I(-) (3)), which can be considered as a [Pd(3)X(3)](3-) fragment flanked by two ring-ligands [(C(7)H(7))(2)](2+). The calculations suggest that the [Pd(3)X(3)](3-)-ligand interaction is mainly arising from electrostatic contributions, where the formally zerovalent Pd atoms allows backdonation of charge from the halide X(1-) atoms to the [(C(7)H(7))(2)](2+) ligands, resulting in a net charge of about +0.4 for each Pd atoms that decreases from 1 to 3. The electronic delocalization estimated via the NICS indexes and the ELF function allows us to describe a significant stabilizing sigma-aromaticity at the center of the Pd(3) triangle, which decreases from [Pd(3)Cl(3)](3-) to [Pd(3)I(3)](3-) (1 to 3) due to the softer character of the iodine counterpart, that donates extra charge to the ligands. The calculated electronic transitions via TD-DFT are in reasonable agreement with the experimental data obtained in CH(2)Cl(2) solution, indicating that the most intense transition involves a core-centered [Pd(3)X(3)](3) transition toward the [(C(7)H(7))(2)](2+) ligands, with mainly X(1-) character in the former molecular spinor that is responsible for the variation of the observed lambda(max) according to the variation of X(1-).