Biruthenium Charge-Transfer Complexes: Measurements of Changes in Dipole Moments and Other Electrooptic Properties

Electroabsorption (Stark effect) spectra are reported for the charge-transfer transitions of (NH3)SRuL2+ and [(NH3)5R~]2L4+*5+, where L is pyrazine (pz) or 4,4'-bipyridine (4,4'-bpy). The spectra permit experimental estimates of the susceptibility of the transition dipole moment to an electric field, the change in polarizability (TrAu), and the magnitude of the change in permanent electric dipole moment (IApI) associated with many of the metal-to-ligand and metal-to-metal charge-transfer (MLCT and MMCT, respectively) transitions in these complexes. The observed electroabsorption spectra of the MLCT transitions of the monoruthenium complexes are interpreted as arising predominantly from Au and Ap. When L = pz and 4,4'-bpy, the observed values of IApl are (5.3 0.8)lfand (15.8 * 0.2)/fD, respectively, compared with the values of 16.5 and 27.1 D expected for full charge transfer from the metal to the geometric center of the ligand (fis a local electric field correction). Protonation of the monoruthenium complexes has relatively small effects on the observed Au and Ap when L = 4,4'-bpy, but when L = pz, Au appears to change its sign while Ap virtually disappears. A simple electrostatic model qualitatively accounts for the results and indicates that pyrazine allows a much greater degree of delocalization from the ruthenium than 4,4'-bipyridine whose pyridyl rings are probably not coplanar. The electroabsorption spectra of the MLCT region of the biruthenium complexes are very complicated and not quantitatively interpretable on the basis of current information, though interesting and qualitatively suggestive features appear. For the MMCT transitions in the biruthenium mixed-valence complexes where L is pz and 4,4'-bpy, the observed values of lApl are (0.7 * O.l)/fand (28.5 A 1.5)/fD, respectively, compared with the values of 32.7 and 54.3 D expected fur charge transfer in fully localized complexes. These latter results demonstrate that electronic delocalization between the two metals is essentially complete when the bridging ligand is pyrazine, whereas it is significant but incomplete when the bridge is 4,4'-bipyridine. In all complexes, the angle dependence of the electroabsorption demonstrates that Ap and the other field-interactive molecular properties are parallel to the transition dipole moment, as expected if the transition moment lies along the metal-ligand axis. The electroabsorption spectra of all of the monoruthenium and [(NH3)5R~]2L4+ complexes also show evidence for transitions that are weak or obscured in conventional absorption spectra; possible assignments are discussed within the context of a molecular orbital model.