[Cs©C] and [Na©C]: perfect planar alkaline-metal-centered polyynic cyclo[]carbon complexes with record coordination numbers.

Searching for the maximum coordination number (CN) in planar species with novel bonding patterns has fascinated chemists for many years. Using the experimentally observed polyynic cyclo[18]carbon C and theoretically predicted polyynic cyclo[14]carbon C as effective ligands and based on extensive first-principles theory calculations, we predict herein their perfect planar alkaline-metal-centered complexes Cs©C (1) and Na©C (4) which as the global minima of the systems possess the record coordination numbers of CN = 18 and 14 in planar polyynic species, respectively. More interestingly, detailed energy decomposition and adaptive natural density partitioning bonding analyses indicate that the hypercoordinate alkaline-metal centers in these 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©C (1) and (π-3s)σ, (π-3p)σ, and (π-3d)σ coordination interactions fabricated in Na©C (4) to dominate the overall attractive interactions between the metal center and its cyclo[]carbon ligand. Similarly, alkaline-metal-centered planar Cs©CB (2), Cs©C (3), Na©CB (5), and Na©C (6) have also been obtained with CN = 18, 17, 14, and 13, respectively.