COORDINATION OF LEAD (Pb) IN CRYSTAL STRUCTURES : A CAMBRIDGE STRUCTURAL DATABASE STUDY. Liat Shimoni-Livny+, Charles W. Bock[[daggerdbl]], Jenny P. Glusker+, The Institute for Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA+, The Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 19144, USA[[daggerdbl]]
Lead is a member of the 'b'-class (soft) elements which, along with cadmium and tin, are potentially dangerous to humans. In its ionic state the plumbous ion Pb(II) is generally more stable than the plumbic ion Pb(IV).
This paper will describe the various modes of binding of lead in its different oxidation states as indicated by the results of crystal structure determination of small molecules solved to a high resolution along with ab initio molecular orbital calculations. We analyzed the preferred coordination numbers of lead, the chemical nature of the ligating atoms, and the geometry of its coordination sphere. Lead in the divalent state possesses, like germanium and tin, a lone pair of electrons in its outer shell. Divalent lead (Pb), with two electrons in its 6s orbital, due to its large atomic number (82) shows a relativistic effect1 so that the inner shells contract and these 6s electrons are stabilized and may become an "inert pair." Thus the energy required to remove or interact with the 6s lone pair of electrons is increased, causing low symmetry around the lead ion. We examine here the geometrical and crystal packing consequences of the presence of this lone pair of electrons in the crystal structures of lead compounds found in the Cambridge Structural Database.2
Supported by grant CA-10925 from NIH.
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