Comment on “Realization of the Zn ³⁺ oxidation state” by H. Fang, H. Banjade, Deepika and P. Jena, Nanoscale , 2021, 13 , 14041–14048

Two zinc-boron clusters (ZnBeB₁₁(CN)₁₂ and ZnBeB₂₃(CN)₂₂) reported in a theoretical study by P. Jena and co-workers are reinvestigated using quantum chemistry calculations. The results prove that the zinc atoms in these two clusters retain a normal oxidation state of +2, overturning the conclusion reached in the previous study that a +3 oxidation state is present. The semi-empirical LOBA method points out this contrast, which is demonstrated via various wavefunction analysis approaches. No unpaired electrons are observed on zinc atoms nor is there a spin density difference distribution, revealing that the zinc atoms have a fully occupied 3d¹⁰ electron shell. Density of states studies give the same conclusion, and they further show that zinc atoms adopt an sp²-hybrid type during bonding. From the perspective of energy, we advise that the electron affinity energy is not a reliable way of evaluating the oxidation state. Instead, binding energy calculations and constrained DFT are applicable, and these also support the presence of Zn²⁺. The simulated XPS peaks are consistent with the experimental data for Zn(II) measured in ZnS. Lastly, the ETS-NOCV method is adopted to give insights into the bonding structures between zinc atoms and boron clusters. It is suggested that future theoretical research into similar problems is analyzed more cautiously to avoid potentially misleading other researchers.