The effect of inert dopant ions on spin-crossover materials is not simply controlled by chemical pressure

[Fe(bpp)2][BF4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine) undergoes abrupt thermal spin-crossover (SCO) at 261 K with a small 2–3 K thermal hysteresis. Different compositions of doped materials [FezZn1−z(bpp)2][BF4]2 and [FezRu1−z(bpp)2][BF4]2 (0 < z < 1) show similar broadening of the SCO transition with increased doping, but differ in their effect on the transition temperature. Doping with zinc strongly lowers T½, which is consistent with previous work. In contrast, doping with ruthenium increases T½ to a smaller degree, which cannot be explained by the chemical pressure arguments that are conventionally applied to doped SCO materials. Mechanoelastic simulations imply that different dopants exert opposite effects on the lattice elastic interactions in the material during the SCO transition. Consistent with that, the materials show a complicated dependence of the crystallographic lattice parameters and thermal expansion properties on the iron spin state, for different dopant ions. These changes correlate with small perturbations to the molecular structure of high-spin [Fe(bpp)2]2+, in the presence of dopants with different geometric preferences and conformational rigidities. We conclude the effect of isomorphous dopants on T½ reflects how the dopant influences the coordination geometry of the iron centres, as well as the chemical pressure exerted by the dopant ion size.