KINETICS OF THE TRIS(1,10-PHENANTHROLINE)COBALT(III II) SELF-EXCHANGEREACTION 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 th e anion X- can be accommodated in terms of the full Debye-Huckel equat ion taking ion size parameter a 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-degrees-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 cont rast, the corresponding volume of activation DELTAV(double dagger) sho ws only a small dependence on I (-20.1 +/- 0.4 cm3 mol-1 as I --> 0; c f. +/- 17.6 +/- 0.7 and -16.0 +/- 0.7 CM3 mol-1 for X = Cl and NO3, re spectively at I = 0. 107 mol L-1), and this is accurately calculable f rom Debye-Huckel-based theory (Inorg. Chem. 1990, 29,5017). Within the limits of detection, Co(phen)3(2+) is entirely high-spin in aqueous s olution (mu(eff) = 4.79 mu(B) with THETA = 20 K, 13-85-degrees-C). For Co(phen)3(3+/2+), DELTAV(double dagger) is much more negative (by som e 15 CM3 mol-1) than adiabatic electron-transfer theory can accommodat e, and this appears to be typical of low-spin/high-spin Co(III/II) sel f-exchange reactions such as Co(en)3(3+/2+), except for Co(sep)3+/2+, for which DELTAV(double dagger) conforms well to predictions for adiab atic 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-s pin isomer of the Co(II) complex in highly unfavorable equilibrium wit h the high-spin form or, more likely, nonadiabatic electron transfer i nvolving the Co(II) and Co(III) ground states.