OUTER-SPHERE REDOX REACTIONS IN STERICALLY HINDERED PENTAAM(M)INECOBALT(II) COMPLEXES - A TEMPERATURE AND PRESSURE-DEPENDENCE KINETIC-STUDY

Outer-sphere redox reactions between [Co(RNH2)5(H2O)]3+ and [Fe(CN)6]4- have been studied as a function of pH, temperature and pressure. The effect of the size of the alkyl substituent on the amine, RNH2, has been investigated for both the aqua- and the hydroxo-species in order to establish possible correlations between size and ion-pair formation constant, electron-transfer rate constant, and thermal and pressure activation parameters. The values obtained (at 45-degrees-C and ambient pressure) indicate that the ion-pair formation constant decreases with increasing size of R (75, R = H; 40, Me; 23 dm3 mol-1, Et), whereas the electron-transfer rate constant increases in this direction (0.11, R = H; 9.3, Me; 35 s-1, Et). The activation enthalpies do not change, either with decreasing charge on the cobalt complex or with the size of the amine (87, R = H; 79, Me; 84 kJ mol-1, Et). As for the activation volume, although a slight increase is observed on increasing the size of R (26.5, R = H; 29.4, Me; 33.1 cm3 mol-1, Et), it is clear that solvational changes during electron transfer are mainly responsible for the values obtained.