Intervalence charge transfer (IT) transitions of the mixed-valence binuclear complexes [(NH3)5RuII-4,4'-bipyridine-RuIII(NH3)5]5+  and [trans-(py)(NH3)4RuII-4,4'-bpy-RuIII(NH3)4(py)]5+ as well as the LMCT absorption band of  [(NH3)5RuIII(N,N-dimethylaminopyridine)]3+ have been measured in water as a function of various added �simple� electrolytes.  The observed spectral shifts cannot be explained by dielectric continuum theory, �ion-atmosphere� reorganization, or simple ion-paring equilibria. The shifts do correspond, however, in a systematic way to the identity of the anion of the added salt.  We find that the direction of the shift (red or blue) correlates well with the known �structure making� or �structure making� properties of the added anion on liquid water (as indexed by the Jones-Dole viscometric B coefficient or the known ?GHB values which reflect whether a given ion disrupts or enhances the extended hydrogen bond network of liquid water).  The neutral molecule urea also exerts a small-but-measureable effect. These spectral shifts reveal an important role for how the specifically-hydrated ruthenium-ammine chromophores are embedded in and coupled to the surrounding H-bonded network of the water (see figure) and how added salts evidently modulate this component of the spectroscopic Franck-Condon barrier.  These results present an example of �salt effects� which appear to reflect both �local� and �global� perturbations of water structure as sensed by the chromophore, and it is likely that other strongly-hydrated solutes will be found to manifest this effect as well.

Water structure making/breaking properties of added "simple" salts drive systematic shifts in the spectroscopic ligand-to-metal and intervalence-transfer (Franck-Condon) absorption band energies of aqueous mononuclear and binuclear ruthenium ammine complexes

243rd National Meeting (2012)

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