INSTRUMENTAL, THEORETICAL, AND EXPERIMENTAL ASPECTS OF DETERMINING THERMODYNAMIC AND KINETIC-PARAMETERS FROM STEADY-STATE AND NON-STEADY-STATE CYCLIC VOLTAMMETRY AT MICROELECTRODES IN HIGH-RESISTANCE SOLVENTS - APPLICATION TO THE FAC/MER-[CR(CO)3(ETA-3-PH2PCH2CH2P(PH)CH2CH2PPH2)](+/0) SQUARE REACTION SCHEME IN DICHLOROMETHANE

Instrumental, experimental and theoretical approaches required to quantify the thermodynamic and kinetic aspects of the square reaction scheme relating the fac+/0 and mer+/0 redox couples in the high-resistance solvent dichloromethane, at microelectrodes, under both steady-state and fast scan rate (transient) conditions, are presented. fac+, mer+, fac0, and mer0 represent the facial and meridional isomers of Cr(CO)3(eta-3-Ph2PCH2CH2P(Ph)CH2CH2PPh2) in the oxidized 17 electron (fac+, mer+) and reduced 18 electron (fac0, mer0) configurations, respectively. A computationally efficient simulation method based on the DuFort-Frankel algorithm is readily applied to microelectrodes and enables simulations to be undertaken for both steady-state and transient voltammetry at electrodes of microdisk geometry. The minimal ohmic drop present under steady-state conditions enables a limited set of parameters to be calculated for the square scheme. However, data relevant to species generated as a product of electron transfer have to be determined from the transient voltammetry at fast scans rates. For the latter experiments, a newly designed electrochemical cell was developed along with relevant electronic circuitry to minimize the background current and uncompensated resistance. The cell contains two matched working microelectrodes (one in the test solution and one in the separated electrolyte solution) and a common quasi-reference electrode which passes through both compartments of the cell. It is concluded thal a judicious choice of steady-state and transient techniques, such as those described in this work, are necessary to characterize complex reaction schemes in high-resistance solvents. In the example presented in this paper good agreement between both regions of the microelectrode experiments is obtained, although uncompensated resistance still appears to influence the fast scan rate data and this is Indicated by an apparent faster rate of electron transfer obtained via the use of steady-state voltammetry.