Nature of the Nickel-Halogen Bond in NiX⁺ Compounds (X = F, Cl, Br, I): A Multireference Configuration Interaction and Ab Initio Valence Bond Study

In a recent study (<i>Inorg. Chem.</i> <b>2024</b>, <i>63</i>, 11812-11820), gas-phase cationic NiX⁺ compounds (X = F, Cl, Br, and I) were probed by nickel L-edge XAS, from which it was concluded that NiF⁺ was best described as an ionic [Ni²⁺F^-]⁺ complex while the remaining three compounds were described as covalent Ni(3d⁹)<u>L</u> species (<u>L</u> depicts a ligand-based hole). An abrupt transition from a classical to an inverted ligand field was suggested as responsible for the change in ground-state electronic structures. Herein, the NiX⁺ series is investigated by using MRCI and ab initio VB calculations. Nickel L-edge X-ray absorption spectra were also modeled within a ROCIS formalism. It was found that all four compounds possess normal ligand fields and nominal Ni(3d⁹) electron counts. Furthermore, it was found that the dominant bonding interactions in NiF⁺ and NiCl⁺ are two 2-center/3-electron (2<i>c</i>/3e) bonds, while in NiBr⁺ and NiI⁺ the dominant bonding interactions are one Ni-X covalent bond and one 2<i>c</i>/3e bond. The change in bonding interactions between NiF⁺/NiCl⁺ vs NiBr⁺/NiI⁺ was rationalized by considering the marked decrease in resonance energy that stabilizes the 2<i>c</i>/3e bond for the heavier halide congeners, which results from the disparity in electronegativities between the nickel-center and the heavier halides.