Does Pb<sup>2+</sup> Form Holodirected or Hemidirected\nSolvation Geometries in Water?

The hydration properties of the Pb<sup>2+</sup> ion in\naqueous\nsolution have been investigated by using a synergic approach based\non Classical and Car–Parrinello molecular dynamics (CPMD) simulations\nand extended X-ray absorption fine structure (EXAFS) spectroscopy.\nA definite answer has been given to the main question on the Pb<sup>2+</sup> hydration structure, which concerns the formation of either\nholodirected or hemidirected solvation geometries, such terms referring\nto the arrangements of the ligands that can be directed either throughout\nthe surface of an encompassing globe around Pb<sup>2+</sup> or throughout\nonly part of the globe, respectively. Our CPMD results show that the\nPb<sup>2+</sup> ion in water forms a hemidirected 4-fold cluster with\na well-defined distorted pyramidal geometry. This cluster is directed\nthroughout one side of the first shell globe, while the other side\ncontains either two or three very mobile water molecules that do not\nform a well-defined geometry around the Pb<sup>2+</sup> ion. The Pb<sup>2+</sup> first shell structural arrangement determined from the CPMD\nsimulation was confirmed by the EXAFS experimental results. A homodirected\nPb<sup>2+</sup> first shell complex like the 8-fold SAP structure\nobtained from the classical MD simulations cannot be reconciled with\nthe EXAFS experimental data. These findings represent a significant\nstep forward in the understanding of the solvation chemistry of the\nPb<sup>2+</sup> ion, which is fundamental to improving the efficiency\nof lead removal procedures that are crucial to the safety of water\nresources.