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$\rightarrow $d transitions) are photochemically active. Primary quantum efficiencies were found for various lines to be: 313 m$\mu $, 0$\cdot $365; 365 m$\mu $, 0$\cdot $345; 405 m$\mu $, 0$\cdot $085; 435 m$\mu $, 0$\cdot $06. The quantum efficiency of cobaltous ion formation is twice the primary quantum efficiency. No temperature dependence was detected. Ethyl alcohol (up to 75%) and acetone (up to 60%) did not effect the photochemical quantum yield. The radical C$_{2}$O$_{4}^{-}$ 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 C$_{2}$O$_{4}^{-}$ ion. The thermal reaction was also studied. It was found that the reaction rate could be presented by $\frac{-\text{d}[\text{Co}Ox_{3}^{3-}]}{\text{d}t}$ = k$_{1}$[CoOx$_{3}^{3-}$]+k$_{2}$[H$^{+}$] [CoOx$_{3}^{3-}$]. k$_{1}$ and k$_{2}$ were evaluated as 1$\cdot $62 $\times $ 10$^{18}$ exp (-33 600/RT) s$^{-1}$ and 1$\cdot $77 $\times $ 10$^{19}$ exp (- 32 500/RT) s$^{-1}$ (mol./1.)$^{-1}$ 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.