The measurement of molar volume changes of redox reactions using high-pressure voltammetry

Pressure and temperature effects on metal-to-metal charge transfer in cyano-bridged CoIII–FeIIcomplexes

The effects of pressure and temperature on the energy (Eop) of the metal-to-metal charge transfer (MMCT, FeII-->CoIII) transition of the cyano-bridged complexes trans-[L14CoNCFe(CN)5]- and cis-[L14CoNCFe(CN)5]- (where L14=6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) were examined. The changes in the redox potentials of the cobalt and iron metal centres with pressure and temperature were also examined and the results interpreted with Marcus-Hush theory. The observed redox reaction volumes can mainly be accounted for in terms of localised electrostriction effects. The shifts in Eop due to both pressure and temperature were found to be less than the shifts in the energy difference (DeltaE degrees]) between the CoIII-FeII and CoII-FeIII redox isomers. The pressure and temperature dependence of the reorganisational energy, as well as contributions arising from the different spin states of CoII, are discussed in order to account for this trend. To study the effect of pressure on CoIII electronic absorption bands, a new cyano-bridged complex, trans-[L14CoNCCo(CN)5], was prepared and characterised spectroscopically and structurally. X-Ray crystallography revealed this complex to be isostructural with trans-[L14CoNCFe(CN)5].5H2O.

Reflections on the outer-sphere mechanism of electron transfer

The quantitative efficacy of the Stranks–Marcus–Hush theory of volumes of activation ΔV‡ for outer-sphere electron transfer between metal complexes in solution is assessed. The theory predicts ΔV‡ accurately for several couples in aqueous solution, but is satisfactory for polar nonaqueous solvents only at pressures of ca. 100 MPa and above, and accuracy is not improved when the molecular nature of the solvent is allowed for through the Mean Spherical Approximation approach. At low pressures, the calculations become numerically unstable when the isothermal compressibility of the solvent is high and its relative permittivity is low, particularly for the more highly charged couples. For aqueous systems, departures from the predicted ΔV‡ afford insights into the role of the counterions, the incursion of inner-sphere pathways, the enhanced reactivity of CoIII/II cage complexes relative to conventional chelates, and the question of "spin forbiddenness" of electron transfer processes that involve a large change in spin multiplicity. Key words: redox kinetics, inorganic reaction mechanisms, pressure effects, Marcus–Hush theory, activation volumes.

Separation of Intrinsic and Electrostrictive Volume Effects in Redox Reaction Volumes of Metal Complexes Measured Using High-Pressure Cyclic Staircase Voltammetry

Redox reaction volumes, obtained by high-pressure cyclic voltammetry, are reported for a selection tris(diimine), tris(diamine), hexaammine, and hexaaqua couples of Fe(III/II), Cr(III/II), Ru(III/II), and Co(III/II). Separation of the intrinsic and electrostrictive volume contributions for these couples has been achieved, some in both aqueous and acetonitrile solutions. For the Co(phen)(3)(3+/2+) system, the intrinsic volume change is estimated to be +15.3 +/- 2.1 cm(3) mol(-)(1) (based on measurements in water) and +16.5 +/- 2.0 cm(3) mol(-)(1) (in acetonitrile). For the Co(bipy)(3)(3+/2+) system, values are +12.7 +/- 1.4 cm(3) mol(-)(1) (in water) and +15.5 +/- 2.5 cm(3) mol(-)(1) (in acetonitrile). Using these experimentally determined intrinsic contributions, a simple structural model suggests that the intrinsic volume change for these reactions can be described using the change in effective volume of a sphere with radius close to that of the coordinating-atom-metal bond length. Electrostrictive volume changes for the 3+/2+ complex-ion couples are a function of solute size and coordinated ligands. For Ru(H(2)O)(6)(3+) and Fe(H(2)O)(6)(3+) reduction, volume behavior is significantly different from that of the other systems studied and can be rationalized in terms of possible H-bonding interactions with surrounding solvent which affect the electrostrictive volume changes but which are not available for the ammine and other complexes studied.

Kinetics and mechanism of the oxidation of glutathione by hexacyanoferrate (III) in aqueous solution

A detailed kinetic study of the oxidation of glutathione by Fe(CN)6(3)- was performed as a function of pH, temperature, and pressure. The pH profile indicates a maximum pH independent rate in the pH range 5 to 8, which is ascribed to the oxidation of the monoanionic form of glutathione. The activation parameters for the oxidation process in this pH range are characterized by significantly negative delta V# (-22 cm3 mol-1) values. The latter value is in good agreement with that calculated theoretically on the basis of the Marcus-Hush-Stranks relationships, and can be ascribed to a large volume collapse associated with the outer-sphere reduction of Fe(CN)6(3-) to Fe(CN)6(4-).