Molecules with pentagonal bipyramidal geometry, featuring strong axial donor sites, often exhibit enhanced single-ion magnet properties by minimizing quantum tunneling of magnetization (QTM) and extending relaxation times via higher excited states. In such cases, shorter LnO bonds in the equatorial plane, compared to the longer axial bonds, can induce significant transverse anisotropy, which in turn increases the strength of QTM.

Two mononuclear lanthanide complexes, [(L)Ln^III(Ph₃PO)Cl]·CH₃OH] [Ln^III = Dy (1) and Er(2)], are isolated employing a pyridine-based ligand, [2,2′-{(1E, 1′E)-pyridine 2,6–diylbis(methaneylylidine}bis(azaneylylidine)diphenol] (H₂L) 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 1are 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.