| Summary: | We present an extensive study to determine the relationship between structural features of spin crossover (SCO) systems based on N-(8-quinolyl)salicylaldimine (qsal) ligand derivatives and their magnetic properties. Thirteen new compounds with general formula [Fe<sup>III</sup>(5-X-qsal)<sub>2</sub>]<sup>+</sup> (X = H, F, Cl, Br and I) coupled to Cl<sup>−</sup>, ClO<sub>4</sub><sup>−</sup>, SCN<sup>−</sup>, PF<sub>6</sub><sup>−</sup>, BF<sub>4</sub><sup>−</sup> and BPh<sub>4</sub><sup>−</sup> anions were prepared and magnetically characterized. The structure/properties correlations observed in these compounds were compared to those of salts with the same [Fe<sup>III</sup>(qsal-X)<sub>2</sub>]<sup>+</sup> cations previously reported in the literature. These cations favour the LS configuration in compounds with the weakest connectivity. As connectivity increases most of them present HS states at room temperature and structures may be described as arrangements of parallel layers of interacting cation dimers. All the compounds based on these cations undergoing complete SCO transitions within the 4–300 K temperature range have high intralayer connectivity. If, however, the interlayer connectivity becomes very strong they remain blocked in the HS or in the LS state. The SCO transition may be affected by the slightest change of solvent molecules content, disorder or even crystallinity of the sample and it remain difficult to predict which kind of ligand substituent should be selected to obtain compounds with the desired connectivity.
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