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(II I/II). Separation of the intrinsic and electrostrictive volume contrib utions for these couples has been achieved, some in both aqueous and a cetonitrile solutions. For the Co(phen)(3)(3+/2+) system, the intrinsi c 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 acetonit rile). 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 si mple structural model suggests that the intrinsic volume change for th ese reactions can be described using the change in effective volume of a sphere with radius close to that of the coordinating-atom-metal bon d length. Electrostrictive volume changes for the 3+/2+ complexion cou ples are a function of solute size and coordinated ligands. For Ru(H2O )(6)(3+) and Fe(H2O)(6)(3+) reduction, volume behavior is significantl y different from that of the other systems studied and can be rational ized in terms of possible H-bonding interactions with surrounding solv ent which affect the electrostrictive volume changes but which are not available for the ammine and other complexes studied.