Ionic Association Effects upon Optical Electron Transfer Energetics: Studies in Water with (CN)5FeII-BPE-FeIII(CN)55−

Robert L. Blackbourn, Yuhua Dong, Andrew Lyon, Joseph T. Hupp

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21 Citations (Scopus)

Abstract

The energetics of optical electron transfer (ET) in (NC)5FeIII-bis(pyridyl)ethylene-FeII(CN)55-(1), as monitored by intervalence absorption spectroscopy, display a remarkable sensitivity to added “inert” electrolyte. With small amounts of added NaCl, CaCl2, or LaCl3, the optical ET (or metal-to-metal charge transfer) energy (EMMCT) increases significantly. With further additions, however, it gradually decreases, ultimately approaching (for LaCl3) the value found in the absence of added electrolyte. The unusual intervalence energy effects are interpreted in terms of stepwise ionic association: unsymmetrical species of the type (NC)5FeIII—BPE—FeII(CN)5−5·Mn+ absorb at higher energies than symmetrical species (i.e., 1 or Mn+⋅1⋅Mn+), because of the existence of a net unfavorable thermodynamic driving force. Support for the interpretation comes from (a) site-specific probes which show that metal cations preferentially associate with the {—FeII(CN)5}3− portion of 1 and (b) calculations of EMMCT (based on experimentally determined ionic association constants and localized energy shifts) which qualitatively reproduce the observed changes in intervalence energetics. Similar intervalence energy behavior is observed for (NH3)5-RuIII—4,4′-bpy-RuII(NH3)55+ with added Na2SO4. This behavior is interpreted in terms of stepwise association of SO42− with the {(NH3)5RuIII—}3+ and {—RuII(NH3)5}2+ sites, respectively, of the polycationic mixed-valence ion. Finally, we speculatively suggest that ion-pairing-induced symmetry reduction also accounts for a most unusual literature observation: the apparent dependence of EMMCT for (NH3)5RuIII—dithiaspirane—RuII(NH3)55+ on the identity and formal potential of the oxidant used to prepare the ion from the 4+ form.

Original languageEnglish
Pages (from-to)4446-4452
Number of pages7
JournalInorganic Chemistry
Volume33
Issue number20
DOIs
Publication statusPublished - Sep 1 1994

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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