Symmetry-breaking structural phase transitions in spin crossover (SCO) complexes play a pivotal role in the development of multifunctional SCO materials. In this work, the influence of counter anions on the symmetry-breaking behavior and SCO properties was investigated in three mononuclear FeIII-qsal type complexes, [Fe(qsal-4-Me)2]A (Hqsal-4-Me = 4-methyl-N-(8-quinolyl)salicylaldimine; A = BF 4 − ( \(\mathbf{1}_{\mathbf{BF}_{\mathbf{4}}}\) ), ClO 4 − ( \(\mathbf{2}_{\mathbf{ClO}_{\mathbf{4}}}\) ), and OTf− (3OTf)). All complexes exhibit a typical “Chain-Layer” structural motif. Single-crystal structural analysis at varying temperatures (50–400 K for \(\mathbf{1}_{\mathbf{BF}_{\mathbf{4}}}\) and 120–300 K for \(\mathbf{2}_{\mathbf{ClO}_{\mathbf{4}}}\) ) revealed distinct symmetry-breaking behaviors in \(\mathbf{1}_{\mathbf{BF}_{\mathbf{4}}}\) and \(\mathbf{2}_{\mathbf{ClO}_{\mathbf{4}}}\) . Notably, \(\mathbf{1}_{\mathbf{BF}_{\mathbf{4}}}\) undergoes complete symmetry-breaking and re-entrant behavior, coupled with a three-step SCO, while \(\mathbf{2}_{\mathbf{ClO}_{\mathbf{4}}}\) shows only symmetry-breaking incomplete SCO without re-entrance. Structure analyses indicated that the more compact crystal packing in \(\mathbf{2}_{\mathbf{ClO}_{\mathbf{4}}}\) strengthens the elastic interactions between the high-spin FeIII centers. This stabilizes the low-spin/high-spin [LS-HS] intermediate phase, thereby inhibiting the reentrant behavior. Compound 3OTf with the largest OTf− anion remains in the HS state below 400 K. While \(\mathbf{1}_{\mathbf{BF}_{\mathbf{4}}}\) showed no LIESST effect, \(\mathbf{2}_{\mathbf{ClO}_{\mathbf{4}}}\) displayed a rare reverse-LIESST effect upon irradiation with 980 nm light. The findings reported here demonstrate that subtle variations in anion type can dramatically modulate SCO behavior, offering valuable insights for the design of functional materials with tailored spin transitions.
