ORIGINAL PAPER Alan M. Bond á Ray Colton á Peter J. Mahon á Wee T. Tan Tetrabutylammonium cation expulsion versus perchlorate electrolyte anion uptake in the electrochemical oxidation of microcrystals of [(C 4 H 9 ) 4 N][Cr(CO) 5 I] mechanically attached to a gold electrode: a voltammetric and quartz crystal microbalance study Received: 4 February 1997 / Accepted: 4 March 1997 Abstract The electrochemistry of microcrystals of [(C 4 H 9 ) 4 N][Cr(CO) 5 I] attached to a gold electrode which is placed in aqueous (lithium or tetrabutyl- ammonium perchlorate) electrolyte media has been studied in detail by chronoamperometric, voltammetric and electrochemical quartz crystal microbalance (ECQCM) techniques. Whilst chronoamperometric and voltammetric measurements show that the expected one-electron oxidation of microcrystalline [Cr(CO) 5 I] ) solid to Cr(CO) 5 I occurs at the solid-electrode-solvent (electrolyte) interface, the ECQCM measurements reveal that charge neutralization does not occur exclu- sively via the expected ejection of the tetrabutyl- ammonium cation. Rather, uptake of ClO 4 ) occurs under conditions where the solubility of sparingly soluble [(C 4 H 9 ) 4 N]ClO 4 is exceeded. This is the ®rst time that uptake of an anion rather than loss of a cation has been detected in association with an oxidation during electrochemical studies of microcrystals attached to electrode surfaces. It is therefore now emerging that analogous charge neutralization processes to those encounted in voltammetric studies on conducting polymers are available in voltammetric studies of mic- rocrystals attached to electrodes which are placed in contact with solvent (electrolyte) media. In the presence of LiClO 4 as the electrolyte, an ion exchange process occurs leading to formation of Li[Cr(CO) 5 I] . X H 2 O which then slowly dissolves in water at a rate that is strongly in¯uenced by the electrolyte concentration, the relatively hydrophobic nature of the [(C 4 H 9 ) 4 N] + cat- ion and the poor solubility of [(C 4 H 9 ) 4 N]ClO 4 . Key words Iodopentacarbonyl chromium(0) [Cr(CO) 5 I] ) á Electrochemical quartz crystal microbalance (ECQCM) á Ion transfer á Voltammetry á Mechanically attached solid Introduction In earlier studies undertaken in these laboratories it was demonstrated that the voltammetry of microcrystalline particles mechanically attached to electrodes and im- mersed in aqueous electrolyte media is dierent from conventional solution studies due to the associated ion transfer processes that accompany the electron transfer process [1±13]. More recently, the use of the electro- chemical quartz crystal microbalance (ECQCM) has proved to be a powerful technique for determining mass changes in the vicinity of the electrode that correspond to these ion transfer processes, and considerable mechanistic information has been obtained [8, 11, 12]. In all the cases examined so far, a simple counter ion of the redox-active species is either expelled or incor- porated into the redox-active microcrystalline solid as a result of the need to maintain electroneutrality throughout the crystal [2, 4±13]. Generally, it has been observed that the mechanism of the process can be determined from knowledge of the counter ion in- volved. When the counter ion is an anion, the general equation for a reduction/expulsion or an oxidation/in- clusion reaction is M z X jzjsolid e M z1 X jz1jsolid X aq 1 and when the counter ion is a cation, we have for a reduction/inclusion or an oxidation/expulsion reaction M z Y jzjsolid Y aq e M z1 Y jz1jsolid 2 where z is the charge of the redox-active species, M. The nature and concentration of the counter ion can strongly in¯uence both the thermodynamic and kinetic aspects of J Solid State Electrochem (1997) 1: 53±61 Ó Springer-Verlag 1997 A.M. Bond (&) á R. Colton á P.J. Mahon Department of Chemistry, Monash University, Clayton 3168, Victoria, Australia Wee T. Tan Chemistry Department, Universiti Pertanian Malaysia, 43400 UPM, Serdang, Selangor D.E., Malaysia