In this work, a theoretical method for the prediction of both the relaxation time of quantum tunneling of magnetization (τ_QTM) and the effective barrier of magnetic reversal (U_eff) is proposed for single-ion magnet (SIM) systems of Kramers type. The reliability of theoretical τ_QTM is tested within a large series of 18 lanthanide SIMs. Compared to the experimental results, the deviations of theoretical τ_QTM are within one order of magnitude for 11 tested SIMs and the largest order-of-magnitude deviation is only 1.86. In the aspect of U_eff, for 5 typical high-performance Dy-SIMs of the local coordination mode of a pentagonal bipyramid, the relative deviations of theoretical values lie within the range of 1.4–7.2%. Thus this method possesses good reliability, at least in the aspect of the order of magnitude. Besides an empirical estimate of the local magnetic field experienced by the central ion, for a given SIM, one ab initio calculation, providing accurate g-factors of both ground and excited Kramers doublets (KDs), is the only computational cost. Therefore this method has a high degree of both reliability and efficiency. Based on the temperature dependence of theoretically predicted U_eff and its contributions from various KDs, some mechanistic information on the magnetic relaxation could be given by this method too. Therefore it is reasonable to expect the bright prospect of this method in the aspects of both the interpretation of the existing experimental results and rational design of future high-performance SIMs.

In this work, a large excess of electrostatic repulsion, arising from the axial ligands, over that from the equatorial ligands is taken as the design strategy for high performance pentagonal bipyramidal (PBP) Dy^III single-ion magnets (SIMs).

Two chromium(ii) complexes exhibit single-molecule-magnet behavior due to uniaxial magnetic anisotropy revealed by magnetic and HF-EPR studies.

Two mononuclear trigonal-planar Co(II) complexes [Na(THF)₆][Co(OAr)₃] 1 and [(THF)₃NaCo(OAr)₃] 2 (OAr⁻ = 2,6-di-tert-butylphenoxo, THF = tetrahydrofuran) with the same coordination number and donor atoms, as well as similar ligand fields except only for the local symmetry of Co(II), were isolated to test the symmetry-magnetization correlations. Although both complexes share similar magnetic anisotropy of the central Co(II) ions (D = −85.4 cm⁻¹ for 1 and D = −80.6 cm⁻¹ for 2), complex 1 exhibits single-ion magnet (SIM) behaviour while 2 displays no slow magnetic relaxation of magnetization with an applied magnetic field up to 7000 Oe. Such distinct performance is ascribed to the different effects of quantum tunneling of magnetization (QTM), which is further associated with the structural symmetry, namely, a strict C_2v local symmetry for 1 and C_s local symmetry for 2. Theoretical calculations also indicate a larger value of transversal factors for 2, and hence a stronger QTM to be suppressed with a larger magnetic field at which the direct process is probably promoted, leading to the absence of SIM behavior of 2.