PRESSURE EFFECT ON THE KINETICS OF THE HEXAAQUAIRON(II/III) SELF-EXCHANGE REACTION IN AQUEOUS PERCHLORIC-ACID

The effect of pressure on the rates of the Fe(H2O)63+/2+ and the Fe(H2O)5OH2+/Fe(H2O)62+ exchange reactions in homogeneous aqueous perchlorate solution can be represented by mean volumes of activation of −11.1 ± 0.4 and +0.8 ± 0.9 cm3 mol−1, respectively, over the range 0–140 MPa at 2 °C and ionic strength 0.5 mol L−1. These results can be accounted for essentially quantitatively on the basis of the Marcus-Hush theory, with minor modifications to accommodate the pressure dependences of the Fe-Fe separation and presumed anion-cation pairing, on the basis of an adiabatic outer-sphere mechanism for the Fe(H2O)63+/2+ exchange and a hydroxide-bridged inner-sphere mechanism for the Fe(H2O)5OH2+/Fe(H2O)62+ reaction. For the Fe(H2O)63+/2+ self-exchange, the question of possible ligand interpenetration in the precursor complex is not unequivocally resolved, but neither aqua-bridged inner-sphere nor markedly nonadiabatic outer-sphere mechanisms are consistent with these measurements. © 1990, American Chemical Society. All rights reserved.