Easy-axis magnetic anisotropy in tetragonally elongated cobalt(II) complexes beyond the spin-Hamiltonian formalism.

Two hexacoordinate Co(II) complexes [Co(hfac) 2 (etpy) 2 ] (1) and [Co(hfac) 2 (bzpyCl) 2 ] (2) were synthesized and spectrally and structurally characterized. The {CoO 4 N 2 } chromophore adopts a geometry of the elongated tetragonal bipyramid with a small o -rhombic component. This less common arrangement causes the magnetic data to need be analysed using the Griffith-Figgis model, instead of the commonly used spin-Hamiltonian with zero-field splitting parameters D and E . In the case of the elongated bipyramid for d 7 complexes, the source of the magnetic anisotropy of an easy-axis type is the axial crystal field splitting Δ ax . The ab initio CASSCF calculations followed by the NEVPT2 module confirm that the ground electronic term is quasi-degenerate owing to the splitting of the 4 E g ( D 4h ) mother term. The lowest spin-orbit multiplets appear as four Kramers doublets belonging to the Γ 5 irreducible representation of the double point group D 2 '. They exhibit a serious mixing of the |±1/2〉 and |±3/2〉 spins which reflects a sizable effect of the spin-orbit coupling. Both complexes exhibit field-supported slow magnetic relaxation governed by the Raman process.