Unexpected isotope effect on the spin transition of the coordination polymer Fe(C5H5N)2[Ni(CN)4]Dedicated to Patrick Cassoux on the occasion of his retirement.

The two-dimensional coordination polymer spin-crossover compound Fe(pyridine)2[Ni(CN)4] (1) and its isotope substituted analogues Fe(pyridine-D5)2[Ni(CN)4] (2) and Fe(pyridine-15N)2[Ni(CN)4] (3) have been synthesised and the isotope effect on the thermal spin-crossover behaviour has been studied using magnetic susceptibility, 57Fe Mössbauer, Raman and calorimetric techniques. All these methods confirmed that upon isotope substitution—contrary to previous observations—the spin transition temperature shifted downwards from 202 to 194 K. The theoretical analysis of spectroscopic data has revealed that this shift was a result of a subtle balance between different vibrational and electronic factors.