Charge-transfer disproportionation

U - A is reduced significantly by the intraatomic-exchange stabilization Ae. Where w = U, this energy is reduced still further by screening, and a charge-transfer disproportionation reaction becomes possible. This most favorable situation applies in the case of CaFeOa-... 

The donor-induced disproportionation in equation (91) leads to the EDA complex, i.e., [D, NO+]NO as the (first) directly observable intermediate. The critical role of the nitrosonium EDA complex in the electron-transfer activation in equation (92) is confirmed by the spectroscopic observation of the cation-radical intermediates (i.e., D+ ) as well as by an alternative (low-temperature) photochemical activation with deliberate irradiation of the charge-transfer band252 (equation 95). 

The study of Li28 + DMF solutions [60] also allowed to characterize the electrochemical properties of polysulfides only redox couples of the type 8 /8 are involved. The chemical reactions coupled to charge transfers are classical dissociation and disproportionation equilibria no complex rearrangement reaction or transient species has been necessary. Redox potentials and charge-transfer coefficients of the redox couples involved in sulfur and polysulfide solutions are summarized in Table 2. 

Moreover, the current-potential curves are affected by the disproportionation reaction therefore, other variables (the rate constant for the disproportionation reaction) must be taken into account. Since experimental results for many interesting systems show clear evidence of slow kinetics, ad hoc simulation procedures have typically been used for the analysis of the resulting current-potential curves [31, 38, 41, 48]. As an example, in reference [38], it is reported that a clear compropor-tionation influence is observed for an EE mechanism with normal ordering of potentials and an irreversible second charge transfer step. In this case, the second wave is clearly asymmetric, showing a sharp rise near its base. This result was observed experimentally for the reduction of 7,7,8,8-tetracyanoquinodimethane in acetonitrile at platinum electrodes (see Fig. 3.20). In order to fit the experimental results, a comproportionation rate constant comp = 108 M-1 s-1 should be introduced. 

Finally, care must be exercised in the photochemical activation of contact ion pairs to irradiate only the charge-transfer absorption bands, and not those of the (colored) products. For example, the irradiation of either Cp2Co+ Co(CO)4 or Q+ Co(CO)4 in the presence of PBu3 at wavelengths beyond 510 nm leads only to the dimeric Co2(CO)6(PBu3)2, despite the fact that the CT photochemistry of the same solution at wavelengths below 550 nm leads smoothly to only the disproportionation products. In fact, control experiments demonstrate that the carbonylcobalt dimer arises from a secondary process by the adventitious excitation of the disproportionation salt, namely,... 

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