Spectra, equilibrium and photoredox chemistry of tri- and tetraiodoplumbate(II) complexes in acetonitrile

The stepwise formation constant of the kinetically labile PbI42- complex was determined in acetonitrile (K-4 = 27 +/- 4 dm(3) mol(-1)). The lowest-energy bands in the absorption spectra of the tri-and tetraiodoplumbate( II) complexes are attributed to ligand-to-metal charge transfer transitions. Photolysis of these complexes in solution results in the formation of Pb(I) and I or I-2(-) as primary products. The quantum yield of this reaction is 0.10 +/- 0.005 for both complexes at 355-nm excitation. The efficiency of the overall reaction (with the formation of I-3(-) as end product) is higher for the PbI3- than for the PbI42- complexes (Phi = 0.026 +/- 0.002 and Phi(4) = 0.010 +/- 0.004) in air-saturated system, lambda(irr) = 366 nm, C-Pb(II) = 1 x 10(-4) mol dm(-3)) due to the different repulsion towards the negatively charged reactants. The overall quantum yield is essentially determined by the competitive dark reactions of the primary products, mostly by those of I-2(-). One key step in the mechanism is the reaction between the I-2(-) intermediate and the ground-state lead(II) complexes, with rate constants of 3.4 +/- 1.1 x 10(7) dm(3) mol(-1) s(-1) and 7.0 +/- 2.0 x 10(7) dm(3) mol(-1) s(-1) at 37 and 94 partial molar percentages of PbI3-, respectively. (C) 1998 Elsevier Science S.A. All rights reserved.