Surface-enhanced Raman spectroscopy of pentaammineosmium (III)/(II) and pentaammineruthenium (II) containing pyridine, pyrazine or 4,4′-bipyridine ligands at silver electrodes: vibrational assignments

Abstract

Detailed vibrational assignments are described for surface-enhanced Raman (SER) spectra of pentaammineosmium (III), (II) and pentaammineruthenium (II) containing pyridine (py), pyrazine (pz) and 4,4′-bipyridine (bpy) ligands adsorbed at the silver—aqueous interface. The assignments are based on group symmetry analysis, deuteration effects and by comparison with corresponding normal Raman and i.r. spectra for the solid-phase complexes and surface and bulk Raman spectra for the corresponding free ligands. Most bands present in the normal Raman and/or i.r. spectra also appear in the SER spectra. This results from the high intensity of the SER spectra along with the relaxation of vibrational selection rules for adsorbed molecules. The appearance of intense SER spectra for Os (NH 3) 5py(III)/(II) and Ru II(NH 3) 5py is noteworthy since, unlike free pyridine, these lack an available nitrogen for surface coordination. The results illustrate the virtues of electrochemical SER spectroscopy for examining the detailed vibrational properties of coordination compounds, including those in oxidation states [such as Os(II) here] that are difficult to obtain pure in bulk media.