ELECTRON-TRANSFER REACTIONS OF SOME DERIVATIVES OF PENTAAMMINE(PYRIDINE)RUTHENIUM(II) AND "PENTAAMMINE(PYRIDINE)RUTHENIUM(III)

The pseudo exchange rate for the pentaammine(pyridine)ruthenium(II)-(III) couple has been determined by studying the rates of the reaction of Ru(NH3)5isn3+ with Ru(NH3)5nic2+ and the reverse. The rate constant is 4.7 × 105 M-1 s-1 at 25 °C in 1.0 M CF3SO3H, and the equilibrium quotient is 2.1, the latter to be compared with 2.5 ± 0.5 as determined from electrochemical measurements. The enthalpy and entropy of activation are 2.9 kcal mol-1 and -23 cal deg-1 mol-1, respectively. Examination of the data on the rates of cross reactions in light of the Marcus correlation reveals no anomalies for reactions among the (pyridine)pentaammineruthenium reactions or for the reactions of the ruthenium complexes with V(II), but, as noted earlier by others, for the Fe3+/2+ couple, the observed rate constants tend to be lower than those calculated. The discrepancy appears to be largely an entropic effect, suggesting the operation of a significant nonadiabatic factor in the Fe3+/2+ couple. © 1979, American Chemical Society. All rights reserved.