Hydration structure and energetic signatures of hemidirected- and holodirected-like Motifs in aqueous Pb2+: A SCAN-QMCF-MD and DLPNO-CCSD(T) study.

The hydration of Pb2+ reflects the combined influence of relativistic 6s stabilization and the stereochemical activity of the 6s2 lone pair, giving rise to coordination environments often described as hemidirected or holodirected. To examine how these structural motifs emerge under ambient aqueous conditions, a 175 ps SCAN-QMCF-MD simulation was combined with DLPNO-CCSD(T) local energy decomposition (LED) analyses on representative first-shell configurations. The SCAN trajectory yields a compact first hydration shell with Pb-O distances centered at 2.37 Å and frequent fluctuations among six-, seven-, and eightfold coordination. Radial and angular distribution functions indicate that the first shell does not adopt a single persistent structure but continuously samples arrangements ranging from hemidirected-like to weakly holodirected. Cluster-based LED analyses of CN = 6 and CN = 8 snapshots reveal a common bonding framework dominated by electrostatics, with non-negligible induction and ligand-to-metal charge transfer. Differences between the two motifs arise primarily from how these energetic components are distributed over different numbers of coordinating ligands rather than from changes in bonding character. The Pb-O vibrational density of states displays a broad stretching band near 294 cm-1, consistent with a compact and dynamically heterogeneous first hydration shell. Together, the structural, vibrational, and energetic trends demonstrate that hydrated Pb2+ explores a continuum of coordination states whose apparent hemidirected or holodirected character reflects the balance between inner-shell packing, ligand number, and short-range electronic stabilization.