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

Abstract

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.