We report the relevance of cation–anion and cation–solvent–anion interactions to the crystal lattice rigidity and the corresponding effect on the SCO behaviour of six FeIII complexes with the hexadentate Schiff base ligand nsal2trien. Four of these compounds were synthesized and structurally and magnetically characterized for the first time. In the most rigid structures, the SCO behaviour is either not allowed or severely hampered. For lower structural connectivity, incomplete transitions are observed within the 4–300 K temperature range. The most flexible structure shows a complete transition. The interactions between the cationic units and the anions were found to be more important than the cation–cation interactions for the SCO phenomenon and for the overall cooperativity related to the abrupt/gradual character of the transition. Computational studies were performed in order to determine the crystal splitting gap Δoct. The calculated values for the HS and LS Δoct are very similar for all compounds suggesting that these parameters have very little influence on the SCO behaviour.