Resonance-Enhanced Raman Scattering in the Near-Infrared Region. Preliminary Studies of Charge Transfer in the Symmetric Dimers (2,2β-bpy)₂ClRu-4,4'-bpy-RuCl(2,2'-bpy)₂^4+/3+/2+, (H₃N)₅Ru-4,4'-bpy-Ru(NH₃)₅^6+/5+/4+, and (NC)₅Fe-4,4'-bpy-Fe(CN)₅^4-/5-/6-

Three symmetrical mixed-valence dimers (and their oxidized and reduced congeners) have been examined in solution by Raman spectroscopy with use of near-infrared excitation (1064 nm, Nd:YAG source). The specific systems were (2,2'-bpy)₂CIRu-4,4'-bpy-RuCl(2,2'bpy)₂^4+/3+/2+, (H₃N)₅Ru-4,4'-bpy-Ru(NH₃)₅^6+/5+/4+, and (NC)₅Fe-4,4'-bpy-Fe(CN)₅^4-/5-/6-. At 1064 nm the excitation source is nearly in resonance with the metal-to-metal or intervalence charge-transfer transition found in each of the mixed-valence ions. Consequently, resonance-enhanced scattering might be expected. From time-dependent Raman scattering theory, this would then provide a basis for a mode-by-mode evaluation of the Franck-Condon factors associated with intervalence charge transfer (J. Am. Chem. Soc. 1989, 111, 1142). For two of the mixed-valence ions, resonance-enhanced Raman scattering indeed is found. (For the third, the decacyano ion, extensive thermal degradation occurs.) Studies of the corresponding fully reduced ions show, however, that the enhancement effects are due not to intervalence excitation but to weakly preresonant metal-to-ligand excitation. Nevertheless, the experiments do serve to indicate the conditions that will likely be necessary in order to observe intervalence enhancement in symmetrical systems.