INTERVALENCE TRANSFER IN MIXED-VALENCE BIFERROCENE IONS

The mixed-valence cations of biferrocene, (Fc-Fc)+, and biferrocenylacetylene, (Fc-C≡C-Fc)+, have been generated in a series of solvents using [(bpy)2Ru(PPh3)Cl](PF6)2 as an oxidant. The mixed-valence complexes display low-energy (near-infrared) absorption bands which are assigned to intervalence transfer (IT) transitions between isolated Fe(II) and Fe(III) sites. Plots of Eop (the energy of the IT band at λmax) vs. (1/n2-1/Ds) where n2 and Ds are the optical and static dielectric constants of the solvent medium, respectively, have been made and the linear relationship predicted by Hush theory has been found. Using the three ions (Fc-Fc)+, (Fc-CλC-Fc)+, and (Fc-CλC-CλC-Fc)+, the expected linear relationship between Eop and 1/d, where d is the Fe-Fe separation, has also been observed. The results suggest that the mixed-valence ions have localized valence sites and that the observed IT properties are consistent with the Hush treatment for optical electron transfer, although slight deviations may exist because of partial electron delocalization. © 1978, American Chemical Society. All rights reserved.