Two-electron transfer for Tl(aq)(3+)/Tl(aq)(+) revisited. Common virtual Tl-II-Tl-III (4+) intermediate for homogeneous (superexchange) and electrode (sequential) mechanisms

Homogeneous and electrochemical two-electron transfers within the TI(aq)(3+)/TI(aq)(+) couple are considered on a common conceptual basis. For the 2 equiv electrochemical reduction of TI(aq)(3) to TI(aq)(+), the intermediate state with a formal reduction potential, E-1* = 1.04 +/- 0.10 V vs the normal hydrogen electrode, was detected, different from the established value of 0.33 V for a TI3+/TI2+ couple. Examination of obtained electrochemical (cyclic voltammetry (CV) and rotating disk electrode techniques, along with the CV-curve computer simulation procedure) and literature data indicate that the detected formal potential cannot be the property of electrode-adsorbed species, but rather of the covalently interacting dithallium intermediate [TI11-TI11](4+) located at the outer Helmholtz plane. The analysis of microscopic mechanisms, based on the recent hypothesis of H. Taube and the Marcus-Hush theory extended by Zusman and Beratan, and Koper and Schmickler, revealed that the homogeneous process most probably takes place through the superexchange inner-sphere two-electron-transfer mechanism, via an essentially virtual (undetectable) dithallium intermediate. In contrast, the electrochemical process occurs through a sequential mechanism, via the rate-determining step of TI(aq)(2+) ion formation immediately followed by activationless formation of the metastable (CV-active) dithallium state. The second electrochemical electron-transfer step is fast, and shows up only in the peak height (but not in the shape) of the observed CV cathodic wave, The anodic wave for a microscopically reverse process of the oxidation of TI(aq)(+) to TI(aq)(3+) cannot be observed within the considered potential range due to the blocking of through-space electron transfer by the competitor process of ion transfer to the electrode.