Structural effects of the Pb2+ 6s2 lone pair activity: Eccentricity

The stereochemical activity of the 6s² lone pair has profound consequences on the coordination chemistry of metal ions like Tl⁺, Pb²⁺ and Bi³⁺. The coordination chemistry of Pb²⁺ is very likely the most extensively investigated among the metal ions with [Xe]4f¹⁴5d¹⁰6s² electronic configuration, allowing for a detailed analysis of the factors that affect the stereochemical activity of the 6s² lone pair. In this review, the X-ray structures of a wide variety of Pb²⁺ complexes were analyzed by approximating their coordination environments to a prolate spheroid. The eccentricity (ε) of the spheroid provides a straightforward measure of the stereochemical activity of the Pb²⁺ lone pair. Holodirected structures are characterized by a rather spherical distribution of the donor atoms around the metal ion, leading to values of ε < 0.05. The stereochemical activity of the lone pair may result in either moderately hemidirected structures, with ε values in the range 0.05–0.15, or strongly hemidirected structures, when ε > 0.15. Hemidirected structures are favored by low coordination numbers and the presence of negatively charged donor atoms. In this work not only have many representative complexes been analyzed to provide a classification of these systems according to their eccentricity, but also a simple tool is provided for coordination chemists, through which they can easily classify their complexes as holodirected or hemidirected.