Superhalogens Among 3d-Metal Compounds: MF₄, MF₆, MF₁₂, and MF₁₈ (M = Sc-Zn)

The ground states of the neutral and anionic tetrafluoride and hexafluoride series of 3d-metal atoms from Sc to Zn were assigned by using a double-check approach in which the pure and hybrid density functional methods were interchangeably used. It was confirmed that all these neutral fluorides are superhalogens except for TiF₄. The electron affinities of the hexafluorides were shown to be consistently higher than those of the tetrafluorides in accordance with the superhalogen conception of the extra electron delocalization over a larger number of the electronegative ligands. In the search for mononuclear fluorides possessing higher electron affinities, we considered the M(F₂)₆^- and M(F₃)₆^- series where M = Sc-Zn. We found that the optimized geometrical structures in both series may be described as MF₆^-- k(F₂), k = 3 and 6, of which the geometry of the MF₆^- core mimics that of the corresponding hexafluoride anion and the F₂ dimers are kept in a bound state by polarizing forces. In these cases, the electron affinity is decreased by tenths of eV with respect to the electron affinity of the core hexafluorides due to a confinement of the extra electron by the F₂ environment.