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 EDDBF 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.