Jahn-Teller Distortions in Pseudo-Octahedral Low-Spin Ni(III) Complexes With O,O or N,N Bidentate Ligands: A DFT...

The Jahn-Teller (JT) effect plays a central role in determining the structure and electronic properties of open-shell transition-metal complexes, yet its reliable theoretical description remains sensitive to the choice of density functional, basis set, and molecular model. In close analogy to the well-established behavior of d⁹ Cu(II) systems, low-spin d⁷ Ni(III) complexes exhibit an E ⊗ e vibronic instability that gives rise to a multidimensional ("Mexican hat") potential energy surface and dynamic Jahn-Teller behavior. In this work, we present a systematic density functional theory investigation of the Jahn-Teller effect in a series of seven pseudo-octahedral Ni(III) complexes bearing bidentate ligands with O,O or N,N donor atoms, of varying size and rigidity. Geometry optimizations initiated from both axially elongated and compressed starting structures were performed under symmetry constraints to map the Jahn-Teller potential energy surface. At the TPSSh/def2-TZVP level, optimizations from elongated and compressed starting points identify elongated structures as minima and compressed structures as first-order saddle points. The presence of multiple symmetry-equivalent elongated minima and low barriers between them is consistent with a dynamically averaged Jahn-Teller distortion rather than a purely static one. The resulting axial-equatorial bond length differences (ΔR ≈ 0.16-0.23 Å) and Jahn-Teller stabilization energies (ΔE_JT ≈ 0.02-0.04 eV) indicate a moderate but systematic preference for axial elongation across the ligand series. Frontier molecular orbital and spin density analyses reveal that the distortion is driven by lifting of the e_g orbital degeneracy in the low-spin d⁷ configuration, with axial elongation stabilizing the predominantly Ni-centered $d_{z^2}$ -based HOMO. Comparative calculations using PW6B95-D3, M06, B3LYP, and OLYP functionals, as well as alternative triple-ζ and double-ζ (def2-SVP) basis sets, confirm that the qualitative Jahn-Teller behavior and ligand-dependent trends are robust, while quantitative differences emphasize the importance of hybrid meta-GGA and dispersion-corrected approaches. Continuum solvation calculations (water, acetonitrile, and dichloromethane) demonstrate that solvent effects do not alter the qualitative Jahn-Teller distortion pattern or energetic ordering. In addition, simplified ligand models are shown to reliably reproduce the essential features of the Jahn-Teller distortion when the immediate coordination environment is preserved, offering a computationally efficient strategy for mechanistic and methodological studies, albeit with limitations for quantitative spectroscopic predictions and for capturing intermolecular effects such as dispersion and crystal packing. Overall, the results establish a consistent electronic-structure description of Jahn-Teller distortions in pseudo-octahedral Ni(III) complexes and highlight both the capabilities and limitations of current DFT approaches for treating vibronically active transition-metal systems.