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.