Magnetic Anisotropy of Mononuclear Ni II Complexes: On the Importance of Structural Diversity and the Structural Distortions

Mononuclear Ni II complexes are particularly attractive in the area of single‐molecule magnets as the axial zero‐field splitting ( D ) for the Ni II complexes is in the range of −200 to +200 cm −1 . Despite this advantage, very little is known on the origin of anisotropy across various coordination ligands, coordination numbers, and particularly what factors influence the D parameter in these complexes. To answer some of these questions, herein we have undertaken a detailed study of a series of mononuclear Ni II complexes with ab initio calculations. Our results demonstrate that three prominent spin‐conserved low‐lying d–d transitions contribute significantly to the D value. Variation in the sign and the magnitude of D values are found to correlate to the specific structural distortions. Apart from the metal–ligand bond lengths, two different parameters, namely, Δ α and Δ β , which are correlated to the cis angles present in the coordination environment, are found to significantly influence the axial D values. Developed magneto–structural D correlations suggest that the D values can be enhanced significantly by fine tuning the structural distortion in the coordination environment. Calculations performed on a series of Ni II models with coordination numbers two to six unfold an interesting observation—the D parameter increases significantly upon a reduction in coordination number compared with a reference octahedral coordination. Besides, if high symmetry is maintained, even larger coordination numbers yield large D values.