[Cs©C₁₈]⁺ and [Na©C₁₄]⁺: perfect planar alkaline-metal-centered polyynic cyclo[n]carbon complexes with record...

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 D_9h C₁₈ and theoretically predicted polyynic cyclo[14]carbon D_7h C₁₄ as effective ligands and based on extensive first-principles theory calculations, we predict herein their perfect planar alkaline-metal-centered complexes D_9h Cs©C₁₈⁺ (1) and D_7h Na©C₁₄⁺ (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©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. Similarly, alkaline-metal-centered planar C_s Cs©C₁₇B (2), C_2v Cs©C₁₇^- (3), C_2v Na©C₁₃B (5), and C_2v Na©C₁₃^- (6) have also been obtained with CN = 18, 17, 14, and 13, respectively.