Universal Single-Ion Physics in Spin-Orbit-Coupled d and d Ions.

We have identified six new 4d and 4d compounds with isolated RuCl octahedra, with formulas (HMA)RuCl·Cl (1; MA = methylamine), (HGly)RuCl·Cl (2; gly = glycine), (HGly)RuCl·2HO (3), (NH)RuCl (4), (HPy)RuCl (5; py = pyridine), and H(4,4'-bpy)RuCl (6; 4,4'-bpy = 4,4'-bipyridine). We find that the temperature-dependent magnetization is well described by single-ion physics in the presence of spin-orbit coupling and negligible superexchange interactions. Further, we find that many compounds in the literature are also well described by single-ion physics, and our results demonstrate the importance of considering single-ion physics when evaluating candidate geometric frustrated magnets in the presence of spin-orbit coupling.