Pressure dependence of the rate of electron transfer between tris(ethylenediamine)cobalt(II) and -(III) in water

Abstract

The thermal self-exchange reaction of Co(en)? and Co(en)?+ in aqueous solution at 65 OC and ionic strength 0.5 mol L-' proceeds by a homogeneous and a minor heterogeneous path, both of which are accelerated by applied pressure (0.1-207 MPa). For the homogeneous path, the logarithmic plot of the bimolecular rate constant k against pressure P is best described as nonlinear, as theory requires, with a volume of activation A P of -20 cm3 mol-] at 0.

Structure of Λ(δλλ)-[Co(en)3]I3(I)2

“…The larger cobalt(II) complex supports an lel 3 geometry of the bidentate ligands around the cobalt center. The Co-N bond distances in [Co(en) 3 ] 2+ average 2.28 Å , significantly longer than the 1.97 Å average in [Co(en) 3 ] 3+ and consistent with the sluggish redox exchange between the complexes (Jolley et al, 1990). In [Co(en) 3 ]I 3 I, the I À ions are located along the quasi-C 2 axis of the [Co(en) 3 ] 2+ complex ion with close hydrogen-bond contacts from N-H protons of 2.91 Å .…”

Solvent Dynamics and Pressure Effects in the Kinetics of the Tris(bipyridine)cobalt(III/II) Electrode Reaction in Various Solvents

“…The apparent failure of eqs 7 and 8 for Co(bpy) 3 3+ /Co(sep) 2+ reaction in water, like that for the Fe(H 2 O) 6 3+ /Ru(en) 3 2+ reaction (Δ E 1/2 = 534 mV), may be taken as evidence for increasing nonadiabaticity as one goes to higher driving forces. Nonadiabatic electron transfer would give slower rate constants (κ ≪ 1) and more negative volumes of activation than expected. Endicott and Ramasami have presented data on the kinetics of reductions of a variety of Co III complexes by Co(sep) 2+ that indicate that the cross reactions are in general nonadiabatic but that κ → 1 as Δ E 1/2 → 0; in other words, self-exchange (and electrode) reactions are adiabatic but kinetic parameters for cross reactions with high Δ E 1/2 may deviate from the predictions of eqs 7 and 8.…”

Comparative Kinetic Analysis of Reversible Intermolecular Electron-Transfer Reactions between a Series of Pentaammineruthenium Complexes and Cytochromec

“…Using the theoretical treatment of Stranks, Swaddle et al., − and Wherland et al., , it is possible to calculate the activation volume from eq 11 According to this theory the activation volume consists of six components: Δ V * IR is the inner-sphere rearrangement, which is neglected here because the contribution is usually close to zero 38,42 in these systems; Δ V * COUL is the Coulombic term; Δ V * DH includes Debye−Hückel or other electrolyte effects; Δ V * SR is the contribution from the solvent reorganization; and β RT is the contribution from the preexponential part of the work terms (=1.3 cm 3 mol -1 ).…”