Photoredox Chemistry of Chloromercurate(II) Complexes in Acetonitrile

Abstract

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.