The spin-orbit effects on platinabenzene: a ring current and electron delocalization approach

The aromatic character of the platinabenzene molecule has been under discussion since it was synthesized in 2002. Previous reports conclude that it has an aromatic character but without total evidence. The lack of all-electron relativistic analysis, the correct electron counting, and the spin-orbit effects were some of the missing points here fulfilled. It was found that platinabenzene is not 6-π electron Hückel aromatic nor 8-π electron Möbius aromatic but 10π electron Hückel aromatic. The aromatic character was tackled from two different points of view. First, from an electronic structure analysis, and second via the local and global magnetic descriptors, i.e., nucleus independent chemical shift (NICS), and magnetically induced current density (MICD) respectively. That exposes the role of the Pt core electrons in the strength of the current density and chemical shifts, as well as the role of the paramagnetic and diamagnetic component of the induced current density. The huge spin-orbit contribution (9.3 nA T⁻¹) to the strength of current density is also explained. Finally from the analysis of the role of 5d orbitals a classical explanation of the aromaticity based on Hückel's 4n + 2 rule with n = 2 was done. This analysis based on the electronic structure was previously not taken into account for this system nor the spin-orbit contribution to the current density.