The vibrational spectra of the cyanide ligand revisited: the nu(CN) infrared and Raman spectroscopy of Prussian blue and its analogues

A careful analysis of the Raman spectra of the M′_x[M(CN) ₆]_y Prussian blue species has enabled a general model for the interpretation of the ν(CN) vibrational spectra. The spectral patterns are derived from those of the metal ions with local O_h symmetry. Two limiting models are discussed. A 'localized mode' model, involving matrix-isolated species, is in much better accord with the observations than a 'factor group' model. The use of the infrared feature as fingerprint of specific M-CN-M′ units arises from the axis-specific nature of individual T _1u modes. The interpretation of the A_1g and E_g Raman features is done in terms of localized vibrations, with involvement of additional energy terms from the lattice motions.