Complete-active-space self-consistent filed spin-orbit (CASSCF-SO) calculations were performed using OPEN MOLCAS package [
31] to further understand magnetic relaxation mechanisms of
1 and
2. Evidently, calculated
g-tensors of the ground Kramers' doublet (KD) for both complexes are
gx = 0.03,
gy = 0.04,
gz = 17.82 for
1 and
gx =
gy = 0.02,
gz = 17.80 for
2 (Tables S3 and S4 in Supporting information), indicating they possess large magnetic anisotropy. Meanwhile, the predominant wavefunction composition (89.5% |±15/2 > for
1 and 98.5% |±15/2 > for
2) and the direction of principal magnetic axes (Figs. S23 and S24 in Supporting information) for the ground KD also verify this. We find that the orientation of easy axis is not exactly along Er-Cl bond for the former or perpendicular to the plane composed of three coordinated O atoms for the latter, existing a certain deviation, which could be caused by the structural distortions compared with the strict local symmetry of
T or
C3v from the perspective of molecular geometric configuration [
32]. For
1, the admixture of wavefunction emerges at the first excited KD (63.8% |±13/2 > + 18.7% |±11/2 > ) and relatively significant transition magnetic moment between this KD arrives at ~10
−1 μB, therefore a fast QTM process will occur at this KD theoretically, making the calculated
Ueff is around 61 cm
−1 (88 K) (Table S9 and Fig. S27a in Supporting information). Nevertheless, non-ignorable transition probability of 1.20 × 10
−2 μB between the ground KD can be also observed, hinting that a non-vanishing QTM may exist. In reality, two-phonon Raman process also participate in the magnetic relaxation in the temperature range of ac measurement at the same time. Computed LoProp charge of donor Cl using the CASSCF wavefunctions is up to −0.8303, and the poor SMM property of
1 can be attributed to the energy destabilization of the prolate Er(Ⅲ) ion induced by rather strong repulsion with axial highly charged Cl
− ion (Fig. S26 in Supporting information) [
19,
33]. Accordingly, with axial ligand eliminated, the coordination environment of
2 containing merely three O atoms is more suitable for Er(Ⅲ) ion: The energy gap between the ground KD and the first low-lying state is greatly improved to 155 cm
−1 (223 K) (Table S4), and the absolute value of axial crystal-field parameters (CFPs) of
2 (
B20 = −3.32 cm
−1,
B40 = 2.37 × 10
−3 cm
−1 and
B60 = −2.81 × 10
−5 cm
−1) are significantly larger than
1 (
B20 = −1.94 cm
−1,
B40 = 1.27 × 10
−3 cm
−1 and
B60 = −6.78 × 10
−6 cm
−1) and smaller magnitude of non-axial CFP (
Bkq,
q ≠ 0) can be observed in
2 (Tables S6 and S7), indicating stronger axial anisotropy in
2. Besides, the admixture of wavefunction in
2 does not emerge until the fourth excited KD (78.5% |±7/2 > ), and the principal magnetization axis of this doublet is also largely tilted from the ground KD. Despite this, transition probability of 1.07 × 10
−1 μB between the second excited KD indicates the occurrence of QTM process in this KD and computed energy barrier of 224 cm
−1 (322 K) (Table S10 and Fig. S27b in Supporting information). Similar to complex
1, although calculated
Ueff value is larger than the experimental value, it is indeed qualitatively reasonable in terms of the order if not considering other existing effects, such as off-resonance phonon modes and hyperfine interactions [
34,
35].