Spectra, equilibrium and photoredox chemistry of iodobismuthate(III) complexes in acetonitrile

The individual absorption spectra and the stepwise formation constants of the kinetically labile BiI4 −, BiI6 3−, BiI7 4− complexes were determined in acetonitrile (K4=1.5±0.3×105 dm3 mol−1, K6=1.3±0.2×107 dm6 mol−2, K7=160±20 dm3 mol−1). Photolysis of these complexes in solution results in the formation of Bi(II) and Iradical dot or I2radical dot − as primary products. The quantum yields of this reaction at 355-nm excitation are 0.057±0.005 and 0.038±0.004 for the tetra- and heptaiodobismuthate(III) complexes, respectively. The efficiency of the overall reaction (with the formation of I3 − or I2 as end products) also depended on the actual coordination number, the irradiation wavelength and the concentration of Bi(III) as well. The overall quantum yield is essentially determined by the competitive dark reactions of the primary products. In deaerated systems, the diiodide species (I2radical dot −) plays a more important role; one key step in the mechanism is the reaction between I2radical dot − and the ground-state bismuth(III) complexes. In air-saturated solutions, the oxidation of the Bi(II) intermediate, leading to the formation of peroxide, is the crucial reaction affecting the final efficiency.