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The thermodynamics and kinetics of electron transfer are examined as a function of aqueous electrolyte and temperature on self-assembled monolayers containing attached redox centers. The monolayere are formed by coadaorbing an alkanethiol with a pendant pyRu(NH& redox center with an alkanethiol with a terminal carboxylic acid onto clean gold surfaces. The formal potential of the attached redox centers tracks the formal potential of a solution analog in all electrolytes (1 M Na2S04, NaC1, NdOs, or NaClO4) and at all temperaturea (5-55 "C). No change in the monolayer structure is discernible in chronoamperometry or Tafel plots over the same range of conditions. The electron transfer kinetics are independent of the identity of the anion in the electrolyte. The temperature dependence of the Tafel plots conforms qualitatively with the predictions of the Marcus model. A comparison of three voltammetric methods (cyclic voltammetry, chronoamperometry, and ac impedance epectroscopy) confirms that one standard rate constant and one reorganization energy coneistently describes the electron transfer kinetics of the majority of the redox centers. IntroductionThe incorporation of redox centers into self-assembled thiolate monolayers on electrodes provides a useful probe for the behavior of the monolayer.l-12 The coverage of the redox centers, which is easily measured by cyclic voltammetry, yields a direct measure of the concentration of the electroactive thiol in the monolayer. The concentration of the electroactive thiol is especially useful for studying the formation of mixed monolayers or the exchange of thiols in the monolayer with thiols in solution. Ion motion through the monolayeP4 and ion pairings can be studied with respect to the position of the redox center in the monolayer.Similarly, the monolayer provides a useful means of controlling spacing between the electrode and the redox center. Electron transfer kinetics can be measured as a function of potential and temperature without the complications of maw transfer. In order for the kinetic measurementa to be meaningful, several criteria which

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