NONAQUEOUS, OUTER-SPHERE ELECTRON-TRANSFER - DELTA-H, DELTA-S, AND DELTA-V FOR A 0/2+ CHARGE-TYPE REACTION

The kinetics of the outer-sphere electron-transfer reaction between hexakis(2,6-diisopropylphenyl isocyanide)chromium(II) tetrafluoroborate, Cr(CNdipp)6(BF4)2, and a series of cobalt(II) clathrochelates of the type CoL3(BX)2 (L = doubly deprotonated diphenylglyoxime (dpg) or 1,2-cyclohexanedione dioxime (nox) and X = C4H9 or C6H5) have been studied as a function of the cobalt(II) concentration, temperature, pressure, and added electrolyte (tetra-n-butylammonium tetrafluoroborate) in acetonitrile. Second-order rate constants for all reactions vary only a factor of 12 from 1.02 x 10(5) to 11.7 x 10(5) M-1 s-1 at 298 K and an ionic strength of ca. 10(-5) M. With the exception of the slowest reaction, these rate constants agree with the Marcus theory predictions based on previously studied reactions between the cobalt clathrochelates and ferrocenes and the directly measured electron self-exchange of Cr(CNdipp)6+/2+. The enthalpies of activation vary from 8 to 4 kcal/mol, and the entropies of activation vary from -8 to -17 cal/(mol K). A pattern previously observed with the cobalt complexes of increasing rate constant with increasing size, due to lowering of the outer-sphere reorganization energy, is not observed here. Increasing the Bu4NBF4 concentration does not affect the rate constant, a result which is in contrast to other reactions studied. Activation volumes for these reactions varied from +2 to +11 cm3/mol at 298 K and an ionic strength of ca. 10(-5) M. This is the first time positive volumes of activation have been measured for outer-sphere reactions that do not involve reactants of opposite charge. Increasing the [BF4-] had little effect on the volume of activation, although a trend of slightly less positive values with increasing salt concentration was observed for the reaction of Co(dpg)3(BPh)2 with Cr(CNdipp)6(BF4)2. Although only three different reactions were measured, a direct linear correlation between the entropy and volume of activation was observed. The results are discussed within the Marcus model and on the basis of expectations of molar volumes from intrinsic volume and electrostriction considerations.