The global minima of thirteen combinations of atoms with formula CE 3 M 3 + (E=S-Te and M=Li-Cs) adopt a planar structure with carbon covalently bonded to three chalcogens and ionically bonded to the three alkali-metals to stabilize the first global minima structures containing planar hexacoordinate carbon atoms. File list (2) download file view on ChemRxiv planar_hexacoordinate_carbon.pdf (854.66 KiB) download file view on ChemRxiv Planar_hexacoordinate_carbon_SI.pdf (5.42 MiB)

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.

The exhaustive exploration of the potential energy surfaces of CE2M2 (E = Si–Pb; M = Li and Na) revealed seven global minima containing a planar tetracoordinate carbon (ptC). The design, based on a π-localization strategy, resulted in a ptC with two double bonds forming a linear or a bent allene-type ECE motif. The magnetic response of the bent ECE fragments support a σ-aromaticity. The bonding analysis indicated that the ptCs form C–E covalent bonds and C–M electrostatic interactions.

Herein, the first global minima containing a planar hexacoordinate carbon (phC) atom are reported. The fifteen structures belong to the CE 3 M 3 + (E = S-Te and M = Li-Cs) series and satisfy both geometric and electronic criteria to be considered as a true phC. The design strategy consisted of replacing oxygen in the D 3h CO 3 Li 3 + structure with heavy and less electronegative chalcogens, inducing a negative charge on the C atom and an attractive electrostatic interaction between C and the alkali-metal cations. The chemical bonding analyses indicate that carbon is covalently bonded to three chalcogens and ionically connected to the three alkali metals.

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 =...