Vibrational Spectroscopy of Cyanide-Bridged, Iron(II) Spin-Crossover Coordination Polymers:  Estimation of Vibrational Contributions to the Entropy Change Associated with the Spin Transition^†

A series of cyanide-bridged, iron(II) spin-crossover coordination polymers of formula Fe(py)₂[M(CN)₄] (M = Pd or Pt; py = pyridine) and Fe(pz)[M(CN)₄]·2H₂O (M = Ni, Pd, or Pt; pz = pyrazine) have been studied by recording the Raman and infrared (IR) spectra of their low-spin (LS) and high-spin (HS) forms in the solid state. Spectral changes upon metal and ligand substitution were used to guide the assignment of vibrational modes. Vibrational entropy changes upon spin transition were analyzed within the harmonic approximation. The results confirm the major contribution of the Fe−ligand vibrations to the overall entropy change associated with the spin transition (∼60%), butcontrary to expectationsreveal significant changes in other intra- and intermolecular vibrations, of possible relevance to the high cooperativity of the spin transition.