No abstract

The nu(CN) vibrational spectra of cyanide groups bridging two metal atoms present a confused picture. Factors relevant to the interpretation of the available data are reviewed. Some mechanisms for frequency change, relative to the corresponding terminal species, are made more quantitative than previously described, and others are highlighted for the first time. The kinematic effect is much less important than previously assumed. It seems that an effect responsible for the major part of the frequency increase upon bridging commonly observed is the cation electric field, together with concomitant relaxation (the IVSE model). However, a contribution may well come from the change in the sigma bonding in the CN unit.

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.