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The IR and Raman spectra of the distorted polymeric octahedral complexes M(py) 2 X 2 (M≡Mn, Fe, Co, Ni, Cu and Cd; X≡Cl and Br; py≡pyridine-h 5 and pyridine-d 5 ) are reported at ambient and low temperatures. For M≡Mn, Fe and Co, these compounds undergo phase transformations (α–γ) in the temperature range 300–90 K. These phase transformations do not affect the coordination sphere around the metal ions significantly, and the metal–ligand vibrations are therefore very similar in the two phases. The lattice modes in the Raman spectra reflect the changes which occur in the crystal lattice when some of the pyridine rings undergo rotation around the M–N axes at the α–γ transition. The γ transition is characterized by sharp, well-defined Raman peaks and the pyridine rotational modes, in particular, are sensitive to the structural changes which occur. Raman spectroscopy can distinguish between the α and γ phases effectively. A full assignment of all the modes is presented and the systematic changes in the positions of the IR and Raman bands across the series of first-row transition metals are shown for both the α and γ phases. © 1997 Elsevier Science B.V. |
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