Abrupt Change from Ionic to Covalent Bonding in Nickel Halides Accompanied by Ligand Field Inversion

The electronic configuration of transition metal centers and their ligands is crucial for redox reactions in metal catalysis and electrochemistry. We characterize the electronic structure of gas-phase nickel monohalide cations via nickel L_2,3-edge X-ray absorption spectroscopy. Comparison with multiplet charge-transfer simulations and experimental spectra of selectively prepared nickel monocations in both ground- and excited-state configurations are used to facilitate our analysis. Only for [NiF]⁺ with an assigned ground state of ³Π can the bonding be described as predominantly ionic, while the heavier halides with assigned ground states of ³Π or ³Δ exhibit a predominantly covalent contribution. The increase in covalency is accompanied by a transition from a classical ligand field for [NiF]⁺ to an inverted ligand field for [NiCl]⁺, [NiBr]⁺, and [NiI]⁺, resulting in a leading 3d⁹ L̲ configuration with a ligand hole (L̲) and a 3d occupation indicative of nickel(I) compounds. Hence, the absence of a ligand hole in [NiF]⁺ precludes any ligand-based redox reactions. Additionally, we demonstrate that the shift in energy of the L₃ resonance is reduced compared to that of isolated atoms upon the formation of covalent compounds.