Kinetics of the Tris(1,10-phenanthroline)cobalt(III/II) Self-Exchange Reaction in Aqueous Solution at Variable Pressure

The marked dependence of the rate constant k₂ for the Co(phen)₃^3+/2+ self-exchange reaction in water on the nature and concentration of the anion X⁻ can be accommodated in terms of the full Debye-Hückel equation taking ion size parameter à as adjustable (520 pm for X = Cl, 360 pm for X = NO₃). A small additional contribution to the exchange rate, most evident at low electrolyte concentrations, appears to come from a heterogeneous pathway. At infinite dilution, k₂ is estimated to be 0.35 L mol⁻¹ s⁻¹ at 25 °C; cf. 6.7 and 10.7 L mol⁻¹ s⁻¹ for X = Cl and NO₃, respectively, at ionic strength I = 0.107 mol L⁻¹. By contrast, the corresponding volume of activation ΔV* shows only a small dependence on I (−20.1 ± 0.4 cm³ mol⁻¹ as I → 0; cf. −17.6 ± 0.7 and −16.0 ± 0.7 cm³ mol⁻¹ for X = Cl and NO₃, respectively at I = 0.107 mol L⁻¹), and this is accurately calculable from Debye-Hückel-based theory (Inorg. Chem. 1990, 29, 5017). Within the limits of detection, Co(phen)₃²⁺ is entirely high-spin in aqueous solution (μ_eff = 4.79 μ_B with θ = 20 K, 13–85 °C). For Co(phen)₃^3+/2+, ΔV* is much more negative (by some 15 cm³ mol⁻¹) than adiabatic electron-transfer theory can accommodate, and this appears to be typical of low-spin/high-spin Co^III/II self-exchange reactions such as Co(en)₃³⁺/²⁺, except for Co(sep)³⁺/²⁺, for which ΔV* conforms well to predictions for adiabatic electron transfer. The behavior of Co(phen)₃^3+/2+ and Co(en)₃^3+/2+ is attributed to either adiabatic electron transfer from a low-spin isomer of the Co^II complex in highly unfavorable equilibrium with the high-spin form or, more likely, nonadiabatic electron transfer involving the Co^II and Co^III ground states.