VOLTAMMETRY, ELECTRON-MICROSCOPY, AND X-RAY ELECTRON-PROBE MICROANALYSIS AT THE ELECTRODE AQUEOUS-ELECTROLYTE INTERFACE OF SOLID MICROCRYSTALLINE CIS-CR(CO)2(DPE)2 AND TRANS-CR(CO)2(DPE)2 AND TRANS-[CR(CO)2(DPE)2]+ COMPLEXES (DPE = PH2PCH2CH2PPH2) MECHANICALLY ATTACHED TO CARBON ELECTRODES

Microcrystalline forms (size range 0.1-10 mum) of cis-Cr(CO)2(dpe)2 (cis0), trans-Cr(CO)2(dpe)2 (trans0), and trans-[Cr(CO)2(dpe)2]+ (trans+) (dpe = Ph2PCH2CH2PPh2) may be mechanically attached to carbon electrodes. The voltammetry of these water-insoluble materials produces exceedingly well defined processes over a wide scan rate range when the electrode is placed into aqueous media containing 0.1 M NaClO4 or 0.1 M KClO4 as the electrolyte. Electron probe microanalysis demonstrates that ClO4- partially covers the edges and the surface of the solid after oxidative electrolysis. This suggests that oxidative voltammetry of the uncharged complex occurs at the crystal-electrode-solution interface to form a perchlorate complex. The redox process observed for the arrays of microcrystalline carbonyl compounds attached to the electrode may be summarized by the following reaction schemes: cis-Cr(CO)2(dpe)2 half arrow right over half arrow left cis-Cr(CO)2(dPe)2+ e- and trans-Cr(CO)2(dpe)2 half arrow right over half arrow left trans-[Cr(CO)2(dpe)2]+ + e- half arrow right over half arrow left trans-[Cr(CO)2(dpe)2]+ + e- with cis-[Cr(CO)2(dpe)2]+ slowly isomerizing to trans-[Cr(CO)2(dpe)2]+. Interestingly,the trans0 complex may be reversibly oxidized to trans+ and trans2+ under most conditions, but not as readily reduced from trans+ back to trans0 if the potential is held for short periods of time at potentials intermediate between the trans+/0 and trans2+/+ processes. This indicates that the presence of a pure trans+ phase hinders reduction; however stepping the potential to a value more negative than the reduction potential of the trans+/trans0 couple and then scanning in the positive potential direction restores the current to its original value. The experimental results are in accord with an electrochemical process that takes place at the solid-solution interface to form a layer of oxidized material. Electron transfer is postulated to occur by electron hopping via self exchange and cross redox reactions with the rate (apparent diffusion coefficient) being dependent on the state of the electrode-compound-solution interface and the surface charge.