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

Abstract

The stepvise formation constant of the kinetically labile PbI 4 2 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 PbI 3 − than for the PbI 4 2 complexes (Φ 3 = 0.026 ± 0.002 and Φ 4 = 0.010 ±0.004) in air-saturated system. λ irr=366 nm, C pb(11)= 1 × 100 −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 × 10 7 dm 3 mol −1s −1 and 7.0±2.0 × 10 7 dm 3 mol −1 s −1 at 37 and 94 partial molar percentages of PbI 3 − . respectively.