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
C2v, has been investigated. Four complexes have the general formula [Ni(bpy)X
2]
n+ (bpy=2,2′‐bipyridine; X
2=bpy ( 1 ), (NCS
?)
2 ( 2 ), C
2O
42? ( 3 ), NO
3? ( 4 )). In the fifth complex, [Ni(HIM
2‐py)
2(NO
3)]
+ ( 5 ; HIM
2‐py=2‐(2‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1
H‐imidazolyl‐1‐hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM
2‐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
gi) 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 NO
3? and bpy or HIM
2‐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.
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