TETRABUTYLAMMONIUM CATION EXPULSION VERSUS PERCHLORATE ELECTROLYTE ANION UPTAKE IN THE ELECTROCHEMICAL OXIDATION OF MICROCRYSTALS OF [(C4H9)(4)N][CR(CO)5I] MECHANICALLY ATTACHED TO A GOLD ELECTRODE - A VOLTAMMETRIC AND QUARTZ-CRYSTAL MICROBALANCE STUDY

The electrochemistry of microcrystals of [(C4H9)(4)N][Cr(CO)(5)I] atta ched to a gold electrode which is placed in aqueous (lithium or tetrab utylammonium perchlorate) electrolyte media has been studied in detail by chronoamperometric, voltammetric and electrochemical quartz crysta l microbalance (ECQCM) techniques. Whilst chronoamperometric and volta mmetric measurements show that the expected one-electron oxidation of microcrystalline [Cr(CO)(5)I](-) solid to Cr(CO)(5)I occurs at the sol id-electrode-solvent (electrolyte) interface, the ECQCM measurements r eveal that charge neutralization does not occur exclusively via the ex pected ejection of the tetrabutylammonium cation. Rather, uptake of Cl O4- occurs under conditions where the solubility of sparingly soluble [(C4H9)(4)N]ClO4 is exceeded. This is the first time that uptake of an anion rather than loss of a cation has been detected in association w ith an oxidation during electrochemical studies of microcrystals attac hed to electrode surfaces. It is therefore now emerging that analogous charge neutralization processes to those encounted in voltammetric st udies on conducting polymers are available in voltammetric studies of microcrystals attached to electrodes which are placed in contact with solvent (electrolyte) media. In the presence of LiClO4 as the electrol yte, an ion exchange process occurs leading to formation of Li[Cr(CO)( 5)I]. X H2O which then slowly dissolves in water at a rate that is str ongly influenced by the electrolyte concentration, the relatively hydr ophobic nature of the [(C4H9)(4)N](+) cation and the poor solubility o f [(C4H9)(4)N]ClO4.