Unraveling σ and π Effects on Magnetic Anisotropy in cis-NiA₄ B₂ Complexes: Magnetization, HF-HFEPR Studies, First-Principles Calculations, and Orbital Modeling

By using complementary experimental techniques and first-principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C_2v , has been investigated. Four complexes have the general formula [Ni(bpy)X₂ ]ⁿ⁺ (bpy=2,2'-bipyridine; X₂ =bpy (1), (NCS^- )₂ (2), C₂ O₄^2- (3), NO₃^- (4)). In the fifth complex, [Ni(HIM₂ -py)₂ (NO₃ )]⁺ (5; HIM₂ -py=2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM₂ -py. Analysis of the high-field, high-frequency electronic paramagnetic resonance (HF-HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from -1 to -10 cm^-1 . First-principles SO-CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3, on one hand, and 4 and 5, on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters (D (axial), E (rhombic), and g_i ) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the e_g -like orbitals and is due to the difference in the σ-donor strength of NO₃^- and bpy or HIM₂ -py, whereas the difference in anisotropy between the two compounds is due to splitting of the t_2g -like orbitals; and 2) the anisotropy of complexes 1-3 arises from the small splitting of the t_2g -like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.