Aromaticity in Ancient Zeise’s Salt

Zeise's salt, K[(C₂H₄)PtCl₃]·H₂O, is historically recognized as the first synthesized organometallic compound. Its bonding is commonly described by the Dewar-Chatt-Duncanson model. We analyzed the [(C₂H₄)PtCl₃]^- anion using magnetic response calculations, molecular orbital decomposition, and energetic criteria. Out-of-plane orbitals contribute minimally, while several in-plane orbitals generate strong diatropic currents, indicating a dominant σ-aromatic character. Isodesmic reaction calculations show that the formation of the C-Pt-C ring is energetically favorable, further supporting delocalization beyond ring strain. Electron delocalization assessed with the EDDB_F method indicates 1.8 |e| delocalized over the C-Pt-C fragment. These results support considering Zeise's salt not only as the first organometallic compound but also as a σ-aromatic metallacycle.