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

Abstract

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.

Electrochemistry at High Pressures: A Review

“…The estimated intrinsic and electrostrictive contributions for a range of couples were studied in aqueous solutions [37,53,54,81] and are summarized in Table 3. It can be seen that the electrostrictive volume change is around 20 cm 3 mol…”

“…Under the assumption that this will be the electrostrictive contribution to DV complex for couples such as Co(phen) 3=2 3 Co(bpy) 3=2 3 , Sachinidis [54] The volume of activations for the electron-transfer step were determined from the effect of pressure (0 ± 1 750 atm) on the exchange current density, i o , at constant overvoltage h according to [58]:…”

“…When significant concentrations of chloride, thiocyanate, or azide ions are present, these anions become coordinated to the metal ion and provide efficient (inner-sphere) bridges [83]. Converse- [54] ly, for Ag , complexation with ammonia, which cannot act as a bridge for electron transfer, increases the volume of activation to 21 cm 3 mol À1 . Table 4 summarizes the rate constants k ex and the corresponding volume of activation DV = el for electrode reactions determined as a function of pressure up to 200 MPa using alternating current (AC) voltammetry by Swaddle and coworkers in their Calgary laboratories [81, 87 ± 94].…”

Kinetics of the oxygen exchange reaction ofOrthopediodic acid in aqueous acidic solution at elevated pressures

“…6 is about 4 cm 3 mol 1 by using eq. 7, considering the radius of HslO6 to be 283 pm [21]. Table 1 Activation parameters for the oxygen exchange reaction of 1-15106 in acidic aqueous solution kox 298 (s t ) AH* (kJ mol ~) AS* (J molaK "1) A V*(cm 3 mol "1) …”

The importance of preferential solvation of the CN ligands in electron- and proton-transfers observed for cis-[Ru(CN)2(bpy)2] under ion bombardment

“…T he specific solvent-solute interaction (hydrogenbonds) in the redox and/or electron-transfer of transition-metal complexes was studied by Sachinidis et al [1]. The electron-transfer, accompanied by large alterations in the solvent structure, was also observed in short-range solvent structure caused by strong hydrogen-bonds between the coordinated ligands and the surrounding solvent molecules [2,3].…”