Universal Single-Ion Physics in Spin–Orbit-Coupled d⁵ and d⁴ Ions

We have identified six new 4d⁴ and 4d⁵ compounds with isolated RuCl₆ octahedra, with formulas (HMA)₄RuCl₆·Cl (<b>1</b>; MA = methylamine), (HGly)₄RuCl₆·Cl (<b>2</b>; gly = glycine), (HGly)₃RuCl₆·2H₂O (<b>3</b>), (NH₄)₂RuCl₆ (<b>4</b>), (HPy)₂RuCl₆ (<b>5</b>; py = pyridine), and H₂(4,4′-bpy)­RuCl₆ (<b>6</b>; 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.