Since their discovery in 1991, single-molecule magnets (SMMs) have received significant attention due to their potential applications in spintronics, data storage and quantum computing. The slow relaxation of applied magnetization that defines SMMs is typically dictated by an energy barrier (U) which arises from the magnetic anisotropy, characterized by the zero-field splitting term (D), of a non-zero spin ground state. Tailoring the size and sign of D therefore represents a promising strategy to design molecules with a large U. Single-ion magnets (SIMs) containing only one metal center are of particular interest due to the possibility of predicting anisotropy based on ligand field theory. However, most transition-metal SIMs are still discovered serendipitously, at least in part both because D relies on subtle geometric effects and accurately predicting a priori the geometry or even stoichiometry of heteroleptic complexes formed from a mixture of ligands remains challenging. Here, for the first time, we show that mechanically chelating ligands represent an untapped platform for the design of SIMs. By focusing on rotaxane-based Co II complexes, we were able to apply established computational approaches for the accurate correlation of magnetic anisotropy and the electronic structure of Co II ions to demonstrate that the geometry of the complexes formed can be predicted with sufficient precision to identify interesting magnetic properties a priori.