Mechanistic Study of the Voltammetry of Nonconducting Microcrystalline cis- and trans-Cr(CO)2(dpe)2 Complexes (dpe = Ph2PCH2CH2PPh2) Mechanically Attached to a Graphite Electrode and Immersed in Different Aqueous Electrolyte Media: Identification by Infrared Spectroscopy of cis-[Cr(CO)2(dpe)2]+ Stabilized at the Electrode–Solid–Solution Interface

Alan M. Bond, Ray Colton, Frank Marken, Jacky N. Walter

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Electrochemical oxidation of microcrystalline, but nonconducting, cis-Cr(CO)2(dpe)2 (dpe = Ph2PCH2CH2PPh2) mechanically attached to a graphite electrode which has been placed in aqueous electrolyte media enables the thermodynamically unstable cis-[Cr(CO)2(dpe)2]+ to be identified for the first time by specular reflectance infrared spectroscopy. That is, kinetic stabilization of the thermodynamically favored cis+ → trans+ isomerization process is achieved at the electrode – solid – electrolyte interface. Detailed studies on the oxidation of microcrystalline trans-Cr(CO)2(dpe)2 to trans-[Cr(CO)2(dpe)2]+ which involve the variation of temperature, scan rate, and electrolyte reveal that “thick”-layer and “thin”-layer processes are present. A zero current extrapolation process enables potentials to be calculated which are independent of crystal size and spacing. Potential data calculated in this manner show virtually no dependence on electrolyte cation but a variation of potential of almost 500 mV when 0.1 M NaF is used instead of 0.1 M NaClO4 as the electrolyte. The correlation of oxidation potential data with free energies of partition for anion transport across a water-dichloroethane interface coupled with voltammetric and spectroscopic data indicate that the oxidation reaction may be summarized by eqs 1–5: (1) transsolid ⇌ trans+solid + e-; (2) cissolid ⇌ cis+solid e- (3) A-solution⇌ A- solid; (4) trans+solid +A-solid⇌ (trans+– A- solid); (5) Cis+ solid + A- solid ⇌ (cis+–A-solid) → (trans+–A-solid). A- denotes the electrolyte anion, and the cis0/+ and trans0/+ nomenclature represents the relevant isomer of [Cr(CO)2(dpe)]0/+ in the appropriate oxidation state. The oxidation processes are believed to be accompanied by swelling of crystals, which may aid the transport of ionic species within the solid.

Original languageEnglish
Pages (from-to)5122-5131
Number of pages10
Issue number12
Publication statusPublished - 1 Dec 1994

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