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Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph3SiO− (Ph3SiO−=anion of triphenylsilanol) and the 2,4‐di‐tBu‐PhO− (2,4‐di‐tBu‐PhO−=anion of 2,4‐di‐tertbutylphenol) ligands combined with the weak equatorial field of the ligand LN6, leads to [DyIII(LN6)(2,4‐di‐tBu‐PhO)2](PF6) (1), [DyIII(LN6)(Ph3SiO)2](PF6) (2) and [DyIII(LN6)(Ph3SiO)2](BPh4) (3) hexagonal bipyramidal dysprosium(III) single‐molecule magnets (SMMs) with high anisotropy barriers of Ueff=973 K for 1, Ueff=1080 K for 2 and Ueff=1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher‐order symmetry SMMs.

Since the last decade, the focus in the area of single‐molecule magnets (SMMs) has been shifting constructively towards the development of single‐ion magnets (SIMs) based on transition metals and lanthanides. Although ground‐breaking results have been witnessed for DyIII‐based SIMs, significant results have also been obtained for some mononuclear transition metal SIMs. Among others, studies based on CoII ion are very prominent as they often exhibit high magnetic anisotropy or zero‐field splitting parameters and offer a large barrier height for magnetisation reversal. Although CoII possibly holds the record for having the largest number of zero‐field SIMs known for any transition metal ion, controlling the magnetic anisotropy in these systems are is still a challenge. In addition to the modern spectroscopic techniques, theoretical studies, especially ab initio CASSCF/NEVPT2 approaches, have been used to uncover the electronic structure of various CoII SIMs. In this article, with some selected examples, the aim is to showcase how varying the coordination number from two to eight, and the geometry around the CoII centre alters the magnetic anisotropy. This offers some design principles for the experimentalists to target new generation SIMs based on the CoII ion. Additionally, some important FeII/FeIII and NiII complexes exhibiting large magnetic anisotropy and SIM properties are also discussed.

Three dysprosium(III) single-molecule magnets (SMMs) with the rare hexagonal bipyramidal geometry have been isolated for the first time. Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph₃SiO^- (Ph₃SiO^- = anion of triphenylsilanol) and the 2,4-di-^tBu-PhO^- (2,4-di-^tBu-PhO^- = anion of 2,4-di-tertbutylphenol) ligands combined with the weak equatorial field of the ligand L^N6, leads to [Dy^III(L^N6)(2,4-di-^tBu-PhO)₂](PF₆) (<b>1</b>), [Dy^III(L^N6)(Ph₃SiO)₂](PF₆) (<b>2</b>) and [Dy^III(L^N6)(Ph₃SiO)₂](BPh₄) (<b>3</b>) hexagonal bipyramidal complexes with high anisotropy barriers of U_eff = 973 K for <b>1</b>, U_eff = 1080 K for <b>2</b> and U_eff = 1124 K for <b>3 </b>under zero applied dc field. <i>Ab initio</i> calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher-order symmetry SMMs. <br>

We generate a new air-stable pseudo-D5h Dy(III) Single-Molecule Magnet (Ueff = 1108 K, TB = 14 K) by combining a weak equatorial ligand field from a macrocyclic LN5 ligand with...