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

Abstract

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 C2v 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 (ΔEorb) 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.