Photoelectron spectroscopy and structural characterization of [EDTA · M(II)]2− ·  <i>n</i> H2O (M = Ni, Cu, Zn; <i>n</i> = 0–2) complexes

We investigated microhydrated transition metal-ethylenediaminetetraacetic acid (EDTA) dianion complexes [EDTA · M(II)]2- · nH2O (M = Ni, Cu, and Zn; n = 1, 2) using cryogenic photoelectron spectroscopy in conjunction with theoretical calculations. The measured spectra of [EDTA · Ni/Zn(II)]2- · nH2O closely resemble those of their corresponding bare dianions, with a systematic shift toward the high electron binding energy side upon stepwise hydration and featuring a hexadentate metal-EDTA binding motif. In contrast, the spectra of hydrated [EDTA · Cu(II)]2- exhibit broadened first detachment bands, implying the coexistence of multiple metal-ligand binding forms, as a consequence of the Jahn-Teller effect associated with its d9 electron arrangement. These findings showcase molecular-level insights into the structural adaptability of EDTA-based supramolecular assemblies comprising transition metal ions, water molecules, and multidentate ligand environments.