Single-molecule magnets (SMMs) show magnetic remanence, and lanthanide (Ln) SMMs have provided the largest effective energy barriers to magnetic reversal (Ueff) and hysteresis temperatures (TH) to date. These advances have been driven by classical electrostatic design criteria to increase magnetic anisotropy, where Dy(III) and Tb(III) SMMs with two mutually transanionic ligands show the largest Ueff from stabilising the largest ±mJ projections in the ground state and destabilising the smaller ±mJ states. Perfectly linear (i.e. with a C axis) Dy(III) and Tb(III) complexes would have pure ±mJ states, which would suppress two-phonon Raman and quantum tunnelling of magnetization (QTM) relaxation processes, giving high TH values. However, any deviation from linearity introduces transverse fields that mix the mJ states, reducing TH. Given the difficulty of isolating linear Ln(III) complexes, we considered that a Dy(III) complex with only a single anionic ligand may show purer ±mJ states than a bent bis-anionic complex and hence show relatively high TH. Indeed, the theoretical [DyO] + cation has been predicted to have Ueff > 2084 cm -1 , but the isolation of such a coordinatively unsaturated complex is inherently challenging due to a combination of large Ln cations and predominantly ionic bonding regimes favouring high coordination numbers. The closest molecular analogues of [DyO] + are endohedral fullerenes containing {Dy2O} fragments, and these compounds show favourable SMM properties, but magnetic relaxation is promoted by interactions of Dy(III) ions with the anionic cages.