Composition of the Lead ( II ) Iodide-Lewis Base Adducts and Structure of 1: 1 Adduct of Lead ( II ) lodide-Tetramethylthiourea

Abstract

Addition reaction products of lead(II) halides with Lewis bases have leen characterized according to their composition and structure. Many adducts form in composition (lead halide): (Lewis base) =1: 1 and 1: 2. However, the structure of 1: 1 adduct with a bidentate ligand apparently differs from that of 1: 1 adduct with an unidentate ligand. Futhermore, there are some different structures for 1: 2 adducts with unidentate ligands.
On the contrary, N, N'-dimethylthiourea forms 1: 1 and 1: 2 adducts, but the other thiourea derivative, tetramethylthiourea, only produces a 1: 1 adduct. In this paper we analyzed the crystal structure of 1: 1 adduct of lead(II) iodide with tetramethylthiourea 'to deduce why the base could not produce 1: 2 adduct. The structure is composed of linear array of Pb atoms bridged by two I and an S atoms. The bridges are alternately made by twøshort Pb-I (3.10-3.19Å) and a long Pb-S (3.50-3.57Å) bonds, and two long Pb-I (3.30-3; 34Å)and a short Pb-S (2.75-2.83Å) bonds along the chain. The three longer bonds locate behind the shorter bonds.
Electron density of the S atom of the thiourea molecule is increased in the presence of four methyl groups, and the 6 s² lone-pair electrons of Pb(II) atom avoid the short Pb-S bond. The Pb atom coordinated by such S atom could not accept another electron rich S atom. Thus, the 1: 2 adduct could not form in the system.