SURFACE-ENHANCED RAMAN-SPECTROSCOPIC STUDY OF THE ADSORPTION AND REDUCTION OF [M(BIPY)(3)](2+) COMPLEXES (M=CO,NI) ON A SILVER ELECTRODE

The spectroelectrochemical behavior of the [M(bipy)(3)](2+) complexes (M = Co, Ni and bipy = 2,2'-bipyridine) adsorbed on a silver electrode has been investigated by surface-enhanced Raman spectroscopy (SERS). The voltammetric measurements have shown that in the case of the [Co(bipy)(3)](2+) complex two redox processes can occur. The first one, reversible, occurs near 0.2 V (SCE) and has been assigned to the Co(II)/Co(III) redox process. The SERS results suggest that the transferred electron on going from Co(III) to Co(II) is localized on a molecular orbital with a main contribution of an atomic orbital of the central ion. The Ni(III)/Ni(II) redox process could not be monitored by SERS. The second redox process, less reversible, is observed for potentials more negative than -1.0 V, for both complexes. The dependence of the Raman frequencies on the applied potentials supports the conclusion that the electron transferred in this process is localized in the bipy ligand. Further evidence for the localization of the transferred electron on the ligand is given by reflectance measurements for both complexes. On the basis of the charge-transfer model for SERS and on the data given by the dependence of SERS excitation profiles on the laser excitation energy, an energy diagram for the adsorbed complexes is presented.