Perfect planar polyynic cyclo[n]carbon complexes [Cs©C18]+ and [Na©C14]+ with alkaline-metal centers exhibiting record coordination numbers and transition metal behaviors

AbstractSearching for the maximum coordination number (CN) in planar species with novel bonding patterns has fascinated chemists for many years. Using the experimentally observed cyclo[18]carbonD9hC18and theoretically predicted cyclo[14]carbonD7hC14as effective ligands and based on extensive first-principles theory calculations, we predict herein their perfect planar alkaline-metal-doped complexesD9hCs©C18+(1) andD7hNa©C14+(4) which, as the global minima of the systems with an alkaline metal atom located exactly at the center, possess the record coordination numbers of CN = 18 and 14 in planar species, respectively. More interestingly, detailed energy decomposition and adaptive natural density partitioning bonding analyses indicate that the hypercoordinate alkaline-metal centers in these σ + π dually aromatic complexes exhibit obvious transition metal behaviors, with effective in-plane (π-6s)σ, (π-7p)σ, and (π-5d)σ coordination bonds formed in Cs©C18+(1) and (π-3s)σ, (π-3p)σ, and (π-3d)σ coordination interactions fabricated in Na©C14+(4) to dominate the overall attractive interactions between the metal center and its cyclo[n]carbon ligand. Similar dually aromatic alkaline-metal-centered planarCsCs©C17B (2),C2vCs©C17-(3),C2vNa©C13B (5), andC2vNa©C13-(6) have also been obtained with CN = 18, 17, 14, and 13, respectively.