We have conducted differential-pulse and square-wave voltammetric studies of salt-induced changes on the redox potentials of both the hexaammineruthenium, Ru(II/III)(NH3)62+/3+ and pentaamminepyridineruthenium, Ru(II/III)(NH3)5py2+/3+ couples in water. Alkali-metal halide salts were added incrementally into 2.0 mM ruthenium solutions in contact with a platinum-disk working electrode over a common range of added salt from 20mM to 1.0M.  A double-junction cell configuration was used so that the effects of liquid junction potentials could be quantitatively removed from the data.  The data reveal a progressive, non-linear cathodic shift upon salt addition with varying shift magnitudes ordered as  I- < Br- < Cl- < F- (and no significant dependence on the identity of the alkali-metal cation).  We have also measured small halide-induced shifts in the MLCT absorption band energy of the RuII (NH3)5py2+ complex ion in water.  Here we find a progressive red shift in the charge-transfer band energy which, though considerably smaller, mirrors the qualitative ordering in the effect size seen by electrochemistry.  Interpretations are offered having to do with possibly-different degrees of ion clustering around the solute complexes as well as the known water-structure modifying properties of the halide anions.  We will show how these factors might be expected to preferentially stabilize Ru(III) redox species relative to Ru(II) and to stabilize the MLCT excited state relative to ground state upon addition of more �structure making� anions like fluoride.

Anion-specific halide salt effects on the redox potentials of the hexaammineruthenium(II/III) and pentaamminepyridineruthenium(II/III) couples in aqueous solution and related effects on the metal-to-ligand charge-transfer spectrum of the pentaamminepyridineruthenium(II) complex ion in water

248th National Meeting (2014)

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