Figure 2. Photoredox cycling of copper cyanide complexes in the presence of hydrogen cyanide 5.

Each turn of the cycle results in the oxidation of two molecules of hydrogen cyanide 5 to cyanogen 11, with the concomitant production of two protons and two hydrated electrons. The photoxidation of the tricyanocuprate(I) species 8 to tricyanocuprate(II) 9 is inferred because the corresponding dicyanocuprate(I) is not photoactive at 254 nm, and, according to stability constant calculations, the tetracyanocuprate(I) is a very minor species³³. Dimerization of 9 to the binuclear complex 10 and reductive elimination of cyanogen 11 giving dicyanocuprate(I) 12 is suggested by the kinetics of the decomposition of copper(II) cyanide complexes³⁴. Finally, conversion of dicyanocuprate(I) 12 to tricyanocuprate(I) 8 is suggested by the aforementioned photoinactivity of 12 and stability constant data, and the fact that a cycle is clearly operative.