Recently, we reported a series of global minima whose structures consist of carbon rings decorated with heavier group 14 elements. Interestingly, these structures feature planar tetracoordinate carbons (ptCs) and result from the replacement of five or six protons (H⁺) from the cyclopentadienyl anion (C₅H₅⁻) or the pentalene dianion (C₈H₆²⁻) by three or four E²⁺ dications (E = Si–Pb), respectively. The silicon derivatives of these series are the Si₃C₅ and Si₄C₈ clusters. Here we show that ptC persists in some clusters with an equivalent number of C and Si atoms, i.e., Si₅C₅, Si₈C₈, and Si₉C₉. In all these species, the ptC is embedded in a pentagonal C₅ ring and participates in a three-center, two-electron (3c-2e) Si-ptC-Si σ-bond. Furthermore, these clusters are π-aromatic species according to chemical bonding analysis and magnetic criteria.

Here, we present evidence that the D_2h M₂C₅^0/2+ (M = Li-K, Be-Ca, Al-In, and Zn) species comprises planar hexacoordinate carbon (phC) structures that exhibit four covalent and two electrostatic interactions. These findings have been made possible using evolutionary methods for exploring the potential energy surface (AUTOMATON program) and the Interacting Quantum Atoms (IQA) methodology, which support the observed bonding interactions. It is worth noting, however, that these structures are not the global minimum. Nonetheless, incorporating two cyclopentadienyl anion ligands (Cp) into the CaC₅²⁺ system has enhanced the relative stability of the phC isomer. Moreover, cycloparaphenylene ([8]CPP) provides system protection and kinetic stability. These results indicate that using appropriate ligands presents a promising approach for expanding the chemistry of phC species.

A systematic exploration of the potential energy surface reveals two global minima with three planar tetra coordinate carbons (ptCs) and two global minima with three quasi-ptCs for E₆C₁₅ (E = Si–Pb) combinations.

Ten planar pentacoordinate boron (ppB) systems are reported, each featuring a pentagonal ring composed of tetrels, pnictogens, or their combination around boron. These structures exhibit double aromaticity (σ and π), consistent with Hückel's 4n+2 rule, as confirmed by magnetically induced current density analysis.

This study introduces systems with planar tetracoordinate group 13–15 atoms (E) in pentagonal C₄H₂E rings bridged by Si or Ge.