Electrochemical kinetics of cyanometalate complexes in aqueous solution athigh pressures

For the aqueous couples Os(CN)6(3-/4) MO(CN)(8)(3-/4-), and W(CN)s(8)(3-/4- ), volumes of reaction Delta V-Ag/AgCl relative to Ag/AgCl/4.0 mol L-1 KCl and volumes of activation Delta V(el)double dagger for the electrode reacti ons are reported. Values of Delta V-Ag/AgCl are consistent with a very smal l increase in the metal-carbon bond length on reduction in each case, the m ain component of Delta V-Ag/AgCl (other than that of the reference electrod e) being electrostrictive solvation change. For media in which the cation i s 0.5 mol L-1 K+ or (for Mo) Na+, Delta V(el)double dagger is strongly posi tive (+9.4 +/-0.7, +7.3 +/- 0.7, and +10.8 +/- 0.4 cm(3) mol(-1) for Os(CN) (6)(3-/4-) MO(CN)(8)(3-/4-), and W(CN)(8)(3-/4-), respectively, at 25 degre es C), whereas the theoretical expectation for a mechanism involving only t he cyanometalate anions is -3 +/- 1 cm(3) mol(-1). For Mo(CN)(8)(3-/4-) in Et4NCl, however, Delta V(el)double dagger is -4.2 +/- 0.2 cm(3) mol(-1). In all cases, Delta V(el)double dagger is just one-half of the corresponding parameter Delta V(ex)double dagger for the homogeneous (bimolecular) self-e xchange reaction of the same couple, giving strong confirmation of the "fif ty-percent rule" (Fu, Y.; Swaddle, T. W. J. Am. Chem. Sec. 1997, 119, 7137) . These and related results are interpreted in terms of a mechanism for bot h electrode and homogeneous electron-transfer reactions of cyanometalates i n which the counterion mediates the electron-transfer process. For alkali m etal cations, partial deaquation to permit this mediation results in positi ve Delta V(el)double dagger values, whereas for tetraalkylammonium counteri ons, there are no aqua ligands to be removed and Delta V(el)double dagger i s "normal".