The photochemistry of complex ions: photochemical and thermal decomposition of the trioxalatocobaltate III complex

The photochemistry of the trioxalatocobaltate III complex was studied. It was shown that both the peak in the ultra-violet region (attributed to electron transfer) and that in the blue (attributed to d->d transitions) are photochemically active. Primary quantum efficiencies were found for various lines to be: 313 mμ, 0.365; 365 mμ, 0.345; 405 mμ, 0.085; 435 mμ, 0.06. The quantum efficiency of cobaltous ion formation is twice the primary quantum efficiency. No temperature dependence was detected. Ethyl alcohol (up to 75%) and acetone (up to 60%) did not effect the photochemical quantum yield. The radical C2O4- is postulated as intermediate capable of reducing mercuric chloride in the course of the reaction. The reaction scheme consists of photo-excitation, primary dark back-reaction, dissociation of excited complex and non-rate-determining oxidation of the C2O4- ion. The thermal reaction was also studied. It was found that the reaction rate could be presented by -d[CoOx3-3]/dt=k1[CoOx3-3]+k2[H+][CoOx3-3] k1 and k2 were evaluated as 1.62 x 1018 exp ( - 33 600/RT) s-1 and 1.77 x 1019 exp ( - 32500/RT) s-1 (mol./l.)-1 respectively. Both the neutral and acid reactions were, however, postulated to proceed through a pseudomonomolecular mechanism involving water molecules with the [H+] ion effecting the level of the transition state. Activation energies are discussed and finally the suitability of the trioxalatocobaltate III complex for chemical actionometry is analyzed.