Single-molecule magnets (SMMs) harbour vast opportunities for potential pioneering applications upon optimization like big data storage and quantum computing. Lanthanides were found to be highly suitable candidates in the design of such molecules, as they intrinsically hold a large unquenched orbital momentum and a strong spin–orbit coupling, warranting a high magnetic anisotropy. An indispensable element in successfully tailoring SMMs is the ligand design. Polyimido sulfur ligands offer a promising choice because the polar S⁺–N⁻-bond facilitates both electronic and geometric adaptability to various f-metals. In particular, the acute N–Ln–N bite angle generates advantageous magnetic properties. The [Ph₂PCH₂S(NtBu)₃]⁻ anion, introduced from [(thf)₃K{Ph₂PCH₂S(NtBu)₃}] (2) to a series of complexes [ClLn{Ph₂PCH₂S(NtBu)₃}₂] with Ln = Tb (3a), Dy (3b), Er (3c), Ho (3d), and Lu (3e), provides tripodal shielding of the metal's hemisphere as well as a side-arm donation of a soft phosphorus atom. For the Tb and Er complexes 3a and 3d, slow magnetic relaxation (U_eff = 235 and 34.5 cm⁻¹, respectively) was only observed under an applied dc field. The dysprosium congener 3b, however, is a true SMM with relaxation at zero field (U_eff = 66 cm⁻¹) and showing a butterfly hysteresis close to 3.5 K. Upon magnetic dilution with the diamagnetic and isostructural lutetium complex 3e or application of a magnetic field, the energy barrier to spin reversal is increased to 74 cm⁻¹.