Spin-orbit effects on electronic delocalization. Aromaticity in a discrete square tetrapalladium sandwich complex

In this article, we report the relativistic electronic structure, including spin-orbit interaction, employing all-electron density functional theory calculations on the multimetallic sandwich compound [(CNT)Pd(4)(COT)](1+) (1), which can be considered as a [Pd(4)](2+) fragment flanked by two ring-ligands, namely, cyclononatetraenyl (CNT(1-)) and cyclooctatetraene (COT), as well as the dimer of 1, hereafter 2. The calculations suggest that the Pd(4)-ligand interaction is mainly electrostatic, being the main responsible term for the stabilization of the almost fully occupied 4d shell [Pd(4)](2+) fragment. The ring currents and electronic delocalization estimated via the nuclear independent chemical shifts indices and electron localization function, allow us to describe a significant sigma-aromaticity at the center of the Pd(4) square in 1, which in conjunction with the aromaticity arising from the ligands induce considerable aromatic character inside of the multimetallic metallocene.