How Do Heavier Halide Ligands Affect the Signs and Magnitudes of the Zero-Field Splittings in Halogenonickel(II) Scorpionate Complexes? A Theoretical Investigation Coupled to Ligand-Field Analysis.

This work presents a detailed analysis of the physical origin of the zero-field splittings (ZFSs) in a series of high-spin (S = 1) nickel(II) scorpionate complexes Tp*NiX (Tp* = hydrotris(3,5-dimethylpyrazole)borate, X = Cl, Br, I) using quantum chemical approaches. High-frequency and -field electron paramagnetic resonance studies have shown that the complexes with heavier halide ligands (Br, I) have greater magnitudes but opposite signs of the ZFSs compared with the chloro congener (Desrochers, P. J.; Telser, J.; Zvyagin, S. A.; Ozarowski, A.; Krzystek, J.; Vicic, D. A. Inorg. Chem.2006, 45, 8930-8941). To rationalize the experimental findings, quantum chemical calculations of the ZFSs in this Ni(II) halide series have been conducted. The computed ZFS using wave-function-based ab initio methods (state-averaged CASSCF, NEVPT2, and SORCI) are in good agreement with the experiment. For comparison, density functional theory was only marginally successful. The ligand-field analysis demonstrates that the signs and magnitudes of the ZFSs are subtly determined by the trade-off between the negative contributions from the (1,3)A1(1e→2e) transitions relative to the positive contributions from the remaining d-d excited states. The term from (1,3)A1(1e→2e) stems from the structural feature that the metal center displaces out of the equatorial plane, and gains the importance when heavier halide ligand is involved.