One of the determining factors in whether single-molecule magnets (SMMs) may be used as the smallest component of data storage, is the size of the barrier to reversal of the magnetisation, U_eff. This physical quantity depends on the magnitude of the magnetic anisotropy of a complex and the size of its spin ground state. In recent years, there has been a growing focus on maximising the anisotropy generated for a single 3d transition metal (TM) ion, by an appropriate ligand field, as a means of achieving higher barriers. Because the magnetic properties of these compounds arise from a single ion in a ligand field, they are often referred to as single-ion magnets (SIMs). Here, the synthetic chemist has a significant role to play, both in the design of ligands to enforce propitious splitting of the 3d orbitals and in the judicious choice of TM ion. Since the publication of the first 3d-based SIM, which was based on Fe(II), many other contributions have been made to this field, using different first row TM ions, and exploring varied coordination environments for the paramagnetic ions.

This short tutorial review covers recent progress in the field of polynuclear cobalt(II)-based complexes, which display slow magnetic relaxation at low temperature. Cobalt(II) single-molecule magnets (SMMs) can display much larger magnetic anisotropies and hence, potentially higher blocking temperatures than SMMs based on ions where the zero-field splitting originates from a second order spin–orbit coupling, such as manganese(III).

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) (1), [Dy^III(L^N6)(Ph₃SiO)₂](PF₆) (2) and [Dy^III(L^N6)(Ph₃SiO)₂](BPh₄) (3) hexagonal bipyramidal dysprosium(III) single‐molecule magnets (SMMs) with high anisotropy barriers of U_eff=973 K for 1, U_eff=1080 K for 2 and U_eff=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.

A cyclic dodecanuclear nickel complex is the first single molecule magnet based on nickel(II) centres; the material is also the highest spin molecule to show resonant quantum tunnelling of magnetisation.