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Recent studies revealed that magnetic molecules with single spin centers showed exciting phenomena related to quantum information processing, such as long quantum coherence times and Rabi oscillations. In this review, we go over these phenomena according to the essential metal ions, from which we can see the development of single-ion magnets as spin qubits is booming, especially quantum coherence times have been significantly enhanced from nanoseconds to hundreds of microseconds in a short period. Hence, the correlations between the molecular structures and quantum coherence are becoming clearer. In this regard, some chemical approaches to designing better spin qubits have been discussed.

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Three mononuclear octahedral Co(II) complexes are reported, [Co(py) 4 (SCN) 2 ] (1), [Co(py) 4 (Cl) 2 ]•H 2 O (2), and [Co(py) 4 (Br) 2 ] ( 3), that exhibit different distortions with compression (1) or elongation (2 and 3) of the axial positions. Easy plane magnetic anisotropy was confirmed by magnetic, HF-EPR, and computational studies for all complexes. Further analyses indicate that both the sign and magnitude of zero-field splitting parameters experience a significant change (D ≥ ±150 cm −1 ) by tuning of the axial and equatorial ligand field strength. Slow magnetic relaxation is observed for all compounds which is dominated by the Raman process involving both acoustic and optical phonons.

Two mononuclear trigonal-planar Co(ii) complexes with a similar coordination environment except for the symmetries were reported to exhibit distinct relaxation dynamics due to the effect of the QTM and direct process.