PRESSURE-DEPENDENCE OF THE RATE CONSTANTS FOR THE ELECTRON ATOM SELF-EXCHANGE BETWEEN M(II)(CP)2 AND M(IV)(CP)2X+ (M = RU, OS X = BR, I) AS A FUNCTION OF SOLVENT

The rate constants for the atom/electron self-exchange between Ru(cp)2 and Ru(cp)2Br+ (cp represents the cyclopentadienide anion), Ru(CP)2 and Ru(cp)2I+, and OS(cp)2 and Os(CP)2I+ were measured by H-1 NMR line-broadening techniques as a function of pressure and solvent. The ranges of the volumes of activation as a function of solvent were from -2.3 +/- 0.7 (C6D5NO2) to -3.9 +/- 0.5 cm3/mol (CD3NO2) for the Ru(CP)2/Ru(CP)2Br(PF6) system, from -3.5 +/- 1.0 (CD3NO2) to -7.7 +/- 1.1 cm3/mol (CD3CN) for the Ru(cp)2/Ru(cp)2I(CF3SO3) system, and from -7.6 +/- 0.9 (CD3CN) to 11.0 +/- 1.0 CM3/mol ((CD3)2CO) for the Os(cp)2/Os(cp)21(CF3SO3) system. The Os(cp)2/OS(CP)2I(CF3SO3) system was also studied for acetone/acetonitrile mixtures, and the volumes of activation were found to vary, in a nonlinear fashion, from -7.7 +/- 0.9 to 15.9 +/- 1.0 cm3/mol. The enthalpy and entropy of activation were also measured for the mixed solvents and were found to be more like the acetone values than the acetonitrile values and were independent of mole fraction. The variation in DELTA-V(double-dagger) and the occurrence of positive values are attributed to a significant contribution to the activation parameters from solvent displacement and a varying degree of solvation between the solvents investigated.