Planar tetracoordinate carbon molecules with 14 valence electrons: examples of CBe4Mnn−2 (M = Li, Au; n = 1–3) clusters

Planar tetracoordinate or pentacoordinate carbon (ptC and ppC) systems are dominated by the 18-electron counting, and a 14-electron ptC cluster is generally considered odd if not impossible. Herein we report computational evidence for a series of ternary CBe4Mnn−2 (M = Li, Au; n = 1–3) clusters, which feature 14 valence electrons and a uniform ptC center. These global-minimum clusters are established via computer global searches, followed by electronic structure calculations at the PBE0-D3, B3LYP-D3, and single-point CCSD(T) levels. In all six species, a core ptC CBe4 unit is stabilized by one to three peripheral Li/Au atoms in a bridging fashion. Chemical bonding analyses reveal delocalized 2π/6σ frameworks around the ptC center, thus rendering double π/σ aromaticity according to the (4n + 2) Huckel rule. The delocalized π/σ frameworks collectively conform to the 8-electron counting and are irrelevant to the “18-electron rule”. The 14-electron ptC clusters can potentially be extended to isoelectronic CBe4Mnn−2 (M = Na, K, Cu, Ag; n = 1–3) systems. Cationic ptC CBe4M3+ (M = Li, Na, K, Cu, Ag, Au) clusters have low vertical electron affinities (2.36–4.77 eV) and belong to the class of exotic species called superalkali or pseudoalkali cations.