DETERMINATION OF THE COMPROPORTIONATION CONSTANT FOR A WEAKLY COUPLED MIXED-VALENCE SYSTEM BY TITRATION OF THE INTERVALENCE TRANSFER BAND - MU-(4,4'-BIPYRIDYL)-BIS(PENTAAMMINERUTHENIUM)(5+)

The change in absorbance in the region of the intervalence transition for μ-4, 4'-bipyridyl-bis(pentaammineruthenium) when the fully oxidized state is titrated with a reducing agent is used to obtain the comproportionation constant for reaction I. This value (24±1 at 19.0°C) was used to calculate the extinction coefficient (8.8X102 M-1 cm-1) at the band maximum (1030 nm in D2O) and the oscillator strength corresponding to the transition. The stabilization resulting from electron delocalization in turn is calculated from the oscillator strength as 48 cal and falls far short of the stability of the mixed-valence compared to the isovalent state, which amounts to 5X102 cal. It seems likely that the major contribution to the stability of the mixed-valence state is the electrostatic interaction between the positively charged ends, which will favor the mixed-valence state. Calculation of this contribution using the macroscopic dielectric constant for the solvent, however, falls far short of accounting for the experimental value. When, as in this case, the contribution to stability from electron delocalization is small, the measurements would seem to provide a rather direct way of exploring the electrostatic term. © 1979, American Chemical Society. All rights reserved.