Ternary CBe₄Au₄ cluster: a 16-electron system with quasi-planar tetracoordinate carbon

Planar hypercoordinate carbons as exotic chemical species are dominated by 18-electron counting. We report herein a 16-electron planar tetracoordinate carbon (ptC) cluster, CBe₄Au₄, which is quasi-planar to be exact, being composed of a C center, a square-planar Be₄ ring, and four outer Au bridges. The quasi-ptC cluster is established as a global minimum via computer structural searches, located 14.6 kcal mol^-1 below the nearest competitor at the CCSD(T) level. It shows thermodynamic and electronic robustness, with a low electron affinity (1.54 eV at B3LYP) and a large HOMO-LUMO gap (2.21 eV for excitation energy). Bonding analyses reveal 2π and 6σ double aromaticity, in addition to four three-center two-electron (3c-2e) Be-Au-Be σ bonds, confirming that 16-electron counting is perfect for the system. We believe that double (π and σ) aromaticity is a general concept that governs planar or quasi-planar carbons, which overrides the 18-electron rule. Competition between quasi-ptC and tetrahedral carbon (thC) isomers in the CBe₄M₄ (M = K, Au, H, Cl) series is also examined, which sheds crucial light on factors that govern the ptC clusters. The present findings offer opportunities for further planar and unconventional molecules.