Direct Spectroscopic Benchmark of Isolated Au(II) Oxidation State: Electronic Structure and Instability of AuCl3–

Characterization of Au(II) oxidation state remains challenging because d9 Au(II) complexes readily disproportionate to Au(I) and Au(III). We report the first high-resolution cryogenic photoelectron spectroscopic characterization of an isolated mononuclear Au(II) halide anion, AuCl3–, combining cryogenic anion photoelectron spectroscopy (PES) with relativistic quantum chemical calculations. The measured adiabatic detachment energy (ADE) and vertical detachment energy (VDE) are 4.63 ± 0.04 and 4.93 ± 0.04 eV, respectively. The VDE shows a near-plateau relative to AuCl2– upon adding an electronegative chlorine ligand, deviating from conventional superhalogen trends. Electronic-structure analysis reveals a high-energy Au 5d σ-antibonding frontier orbital, its unpaired electron axially localized in the Jahn–Teller-distorted C2v structure. Comparative relativistic vs nonrelativistic calculations confirm relativistic effects suppressing the VDE increase from AuCl2– to AuCl3–. The disproportionation reaction 2AuCl3– → AuCl2– + AuCl4– is downhill by 1.48 eV. These results connect relativistic electronic structure, photodetachment energetics, and Au(II)’s electronic lability and disproportionation tendency.