A limiting factor for the application of single-molecule magnets (SMMs) in high-density data storage is the requirement for expensive liquid helium cooling for them to retain magnetic information. For the last two decades, lanthanide (Ln) complexes with highly axial ligand fields have been frequently targeted, as these can provide the highest effective barriers to magnetic reversal (Ueff) by stabilizing the most magnetic and destabilizing the least magnetic mJ states of selected Ln ions along the molecular anisotropy axis. Isolated dysprosocenium cations, [Dy(Cp R )2] + (Cp R = substituted cyclopentadienyl), and related derivatives, have recently provided SMMs with 100 s magnetic blocking temperatures (T100) that are tantalizingly close to what can be achieved with cheaper liquid nitrogen cooling (77 K). The remarkable SMM properties of dysprosocenium cations arise from a combination of their highly axial ligand fields and molecular rigidity, which maximize magnetic anisotropy and inhibit molecular vibrations that promote magnetic relaxation. The largest Ueff values are obtained for [Dy(Cp R )2] + cations when the Cp R ligands are close to the Dy(III) ion and the Cp R centroid …