Photoredox Chemistry of Chloromercurate(II) Complexes in Acetonitrile

The kinetically labile chloromercurates(II) have rather high stepwise formation constants in acetonitrile; even for HgCI42- a value of 1.07 X lo4 M-I was determined. The ultraviolet spectra of these complexes are characterized by ligand-to-metal charge-transfer transitions of different energies and intensities. Their photolysis leads to the formation of Hg(I) and Cl in the primary photochemical reaction. The mercury(I) monomers recombine to give Hg2C12, while chlorine radicals are reconverted to CI-, increasing the Cl-/Hg( 11) ratio in the solution. In considerable excess of CI- ([Cl-] = 5 X 10-3 M at 4.5 x 10-5 M HgC142-) mercury(I) species undergo disproportionation giving HgO as end product. The higher the ligand number of the complex, the lower is the quantum yield for the overall reaction: 5.3 10-2, 1.35 X 10-2 and 9.5x10-3 for HgC12, HgC13-, and HgC142-, respectively, in deaerated solution at λir = 254 nm. In the presence of air quantum yields are lower and the main product is Hg2C12 in all cases. Ethanol enhances the efficiency of the photoreduction but only in argon-saturated systems, indicating competitive reactions of the Hg(I) monomers formed in the primary step and a considerable oxygen sensitivity of HgO as well