ELECTRONIC COUPLING IN CYANO-BRIDGED RUTHENIUM POLYPYRIDINE COMPLEXES AND ROLE OF ELECTRONIC EFFECTS ON CYANIDE STRETCHING FREQUENCIES

The spectroscopic and electrochemical properties of a series of polynuclear complexes containing cyano-bridged ruthenium polypyridine units are consistent with a valence-localized model. In all the Ru(II) forms, the lowest metal-to-ligand charge-transfer (MLCT) states are localized on N-bonded moieties. The photophysical properties at 298 and 77 K indicate that efficient intercomponent energy-transfer processes from C-bonded to N-bonded chromophoric units take place. The mixed-valence forms of these complexes show low-energy absorption bands which are assigned to metal-to-metal charge-transfer transitions involving C-bonded Ru(II) and N-bonded Ru(III) centers. The resonance Raman spectrum of [NC-RuII(bpy)2-CN-RuIII(bpy)2-CN]2+ under near-resonance conditions with the Ru(II) --> Ru(III) charge-transfer band shows enhancement of the bridging cyanide stretching as expected for this type of electronic transition. In the infrared spectra the number of cyanide stretching bands supports the valence-localized model. For the representative [NC-RuII(bpy)2-CN-RuIII(bpy)2-CN]2+ complex, three CN stretches (one bridging, two terminal) are observed. Time-resolved infrared measurements for the MLCT excited state of [NC-RuII(bpy)2-CN-RuII(bpy)2-CN]+ are reported. The excited-state IR spectrum shows features similar to those of the chemically prepared mixed-valence dimer, [NC-RuII(bpy)2-CN-RuIII(bpy)2-CN]2+, strongly suggesting that valence delocalization is not significant in the excited state. The electronic factors affecting the frequency of the bridging cyanide are analyzed by examining the behavior of the mixed-valence ions [M-NC-M'(bpy)2-CN-M]6+/4+ and [NC-M'(bpy)2-CN-M]3+/2+ (M = [Ru(NH3)5]2+/3+; M' = Ru(II), Os(II), Re(I)).