Outer-sphere Redox Reactions in Sterically Hindered Pentaam(m)inecobalt(III) Complexes. A Temperature and Pressure Dependence Kinetic Study

Article abstract of DOI:10.1039/DT9940003159

Outer-sphere redox reactions between (3+) and (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 deg 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.

Full text of DOI:10.1039/DT9940003159

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