Does Pb²⁺ Form Holodirected or Hemidirected Solvation Geometries in Water?

The hydration properties of the Pb²⁺ ion in aqueous solution have been investigated by using a synergic approach based on Classical and Car-Parrinello molecular dynamics (CPMD) simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy. A definite answer has been given to the main question on the Pb²⁺ hydration structure, which concerns the formation of either holodirected or hemidirected solvation geometries, such terms referring to the arrangements of the ligands that can be directed either throughout the surface of an encompassing globe around Pb²⁺ or throughout only part of the globe, respectively. Our CPMD results show that the Pb²⁺ ion in water forms a hemidirected 4-fold cluster with a well-defined distorted pyramidal geometry. This cluster is directed throughout one side of the first shell globe, while the other side contains either two or three very mobile water molecules that do not form a well-defined geometry around the Pb²⁺ ion. The Pb²⁺ first shell structural arrangement determined from the CPMD simulation was confirmed by the EXAFS experimental results. A homodirected Pb²⁺ first shell complex like the 8-fold SAP structure obtained from the classical MD simulations cannot be reconciled with the EXAFS experimental data. These findings represent a significant step forward in the understanding of the solvation chemistry of the Pb²⁺ ion, which is fundamental to improving the efficiency of lead removal procedures that are crucial to the safety of water resources.