Structural Consequences of Different Metal Compositions in the Doped Spin‐Crossover Crystals [FezM1−z(bpp)2][BF4]2 (M = Ni, Zn; bpp = 2,6‐Bis{pyrazol‐1‐yl}pyridine)

Variable temperature crystallographic characterization of [FezZn1‒z(bpp)2][BF4]2 (bpp = 2,6‐bis{pyrazol‐1‐yl}pyridine; z = 0.88, 0.72 and 0.27) and [FezNi1‒z(bpp)2][BF4]2 (z = 0.83, 0.72 and 0.32) is presented. Comparison with previously published data confirms the isothermal unit cell volume change during spin‐crossover (ΔVSCO) behaves differently in the Zn‐ and Ni‐doped crystals. For the FeZn crystals, the relationship between ΔVSCO and z is continuous for z ≥ 0.3 but is steeper than expected, so ΔVSCO ≈ 0 for z = 0.27. In contrast ΔVSCO in the FeNi materials shows only a small variation between 0.83 ≥ z ≥ 0.46, before decreasing more strongly at higher dilution. ΔVSCO in each FeZn crystal is smaller than for its FeNi analogue with a similar composition. As well as the dopant ion ionic radius, the smaller ΔVSCO for the Zn‐doped materials reflects their T½ values, which are lower than for their FeNi counterparts. The contribution of T½ to this behavior is especially evident at high metal dilution.