Planar pentacoordinate germanium stabilized by the 18-valence-electron rule: structural and bonding comparison with...

This study examines the SiE₂M₃²⁺ and GeE₂M₃²⁺ clusters (E = P, As, Sb; M = Ca, Sr, Ba) stabilized by 18-valence-electron (18ve). Twelve global minima exhibiting C_2v symmetry were identified: six featuring a central silicon atom and six with a central germanium atom. All structures satisfy the geometric criteria for planar pentacoordination as defined by the IUPAC coordination concept. BOMD simulations at 298 and 600 K confirmed their kinetic stability. Using the central atom (Si or Ge) and the E₂M₃²⁺ ring as fragments, the EDA-NOCV analysis reveals that the orbital interaction term (ΔE_orb) constitutes the major contribution to stabilization. This is characterized by a dominant s(Si/Ge)-π(ring) coupling, accompanied by π reorganization, with no evidence of an independent Si/Ge-M σ bond. However, IQA analysis reveals that in SiP₂M₃²⁺, all three Si-M interactions are repulsive, resulting in a planar dicoordinate silicon center. In SiAs₂M₃²⁺ and GeP₂M₃²⁺, electrostatic repulsion involving one metal atom (M) prevents the formation of true pentacoordination, resulting instead in planar tetracoordinate centers. Only GeAs₂M₃²⁺ exhibits a genuine planar pentacoordinate germanium center, constituting the first confirmed example of this species stabilized by this specific electron count.