SOLVENT, ANION, AND TEMPERATURE DEPENDENCES OF THE RUTHENOCENE(II) BROMORUTHENOCENE(IV) AND RUTHENOCENE(II) IODORUTHENOCENE(IV) ELECTRON EXCHANGE

The rates of the electron exchange between Ru(cp)2 and Ru(cp)2Br+ (cp represents the cyclopentadienide anion) as the PF6−, C104−, and BF4− salts and between Ru(cp)2 and Ru(cp)2I+ as the CF3SO3− and PF6− salts have been measured by 1H NMR line broadening as a function of temperature and solvent. The observed second-order rate constants at 20 °C and the activation parameters obtained for the Ru(cp)2/Ru(cp)2Br+ system as the PF6− salt are as follows (10−3k in M−1 s−1, ΔH* in kcal mol−1, ΔS* in cal mol−1 K−1): nitrobenzene-d5, 2.9 ± 0.2, 8.1 ± 0.1, −14.8 ± 0.2; nitromethane-d3, 2.5 ± 0.1, 8.5 ± 0.1, -14.1 ± 0.5; acetonitrile-d3, 1.6 ± 0.08, 8.2 ± 0.1, −15.7 ± 0.3; benzonitrile, 1.6 ± 0.08, 8.6 ± 0.3, −14.5 ± 1.1. For the Ru(cp)2/Ru(cp)2I+ system as the CF3SO3− salt, the second-order rate constants at 20 °C and the activation parameters obtained are as follows (10−5k in M−1 s−1, ΔH* in kcal mol−1, ΔS* in cal mol−1 K−1): nitrobenzene-d5, 2.6 ± 0.1, 6.6 ± 0.1, −11.3 ± 0.3; nitromethane-d3, 19.2 ± 1.0, 8.5 ± 0.3, −0.5 ± 0.7; acetonitrile-d3, 7.4 ± 0.4, 6.7 ± 0.2, −8.7 ± 0.8. Only a slight dependence on extra added salt is found for the above solvents. In the low dielectric constant solvents, chloroform-d1, bromobenzene-d5, and methylene-d2 chloride, ion association occurs and thus the observed rate constants and activation parameters are composite quantities from free-ion and ion-paired pathways. For chloroform-d1, the rate constants for two pathways are resolved. The calculations indicate that the electron-transfer reaction does not occur appreciably via the ion-paired pathway. There is no significant dependence of rate on the identity of the counterion. Decamethylruthenocene(II) does not exchange with bromo-or iododecamethylruthenocene(IV) even at 110 °C in nitrobenzene-d5. © 1990, American Chemical Society. All rights reserved.