Insight into Structure: Function Relationships in a Molecular Spin‐Crossover Crystal, from a Related Weakly Cooperative Compound

The ClO 4 – salt of [Fe L 2 ] 2+ { L = 2,6‐bis(3‐methylpyrazol‐1‐yl)pyridine} undergoes very gradual thermal spin‐crossover centered just below room temperature. In contrast, the BF 4 – salt of the same complex exhibits an abrupt and structured spin‐transition at lower temperature, with a complicated structural chemistry. The difference can be attributed to a much larger change in molecular structure between the spin states of the complex in the more cooperative BF 4 – salt, leading to an increased kinetic barrier for their interconversion. Consistent with that suggestion, the high‐spin and low‐spin structures of weakly cooperative [Fe L 2 ][ClO 4 ] 2 are almost superimposable.