Phosphine Oxide Coordinated Mononuclear LnIII(Ln = DyIII, ErIII) Single‐Ion Magnet with Pseudo‐Pentagonal Bipyramidal Geometry

Two mononuclear lanthanide complexes, [(L)Ln III (Ph 3 PO)Cl]·CH 3 OH] [Ln III = Dy ( 1 ) and Er( 2 )], are isolated employing a pyridine‐based ligand, [2,2′‐{(1 E , 1′ E )‐pyridine 2,6–diylbis(methaneylylidine}bis(azaneylylidine)diphenol] (H 2 L) and triphenylphosphine oxide. Both complexes are characterized by single‐crystal X‐ray diffraction studies. Pseudo‐pentagonal bipyramidal geometry (pseudo‐ D 5h ) around the lanthanide centers in both complexes is confirmed through SHAPE analysis. Dynamic magnetic measurements revealed the single‐ion magnet behavior for both complexes. However, the energy barrier ( U eff ) and relaxation time ( τ 0 ) for complex 1 are extracted as U eff = 34.77 K, τ 0 = 8.07 × 10 −7 s (considering both Orbach and Raman processes). These observed phenomena have been corroborated with the aid of CASSCF‐based ab initio calculations.