Five Bonds to Carbon through Tri-Coordination in 
 Al3C3−/0

Abstract

Here, five bonds to carbon through tri-coordination are theoretically established in the global minimum energy isomers of Al3C3− anion (1a) and Al3C3 neutral (1n) for the first time. Various isomers of Al3C3−/0 are theoretically identified using density functional theory at the PBE0-D3/def2-TZVP level. Chemical bonding features are thoroughly analyzed for these two isomers (1a and 1n) with different bonding and topological quantum chemical tools, such as adaptive natural density partitioning (AdNDP), Wiberg Bond Indices (WBIs), nucleus-independent chemical shifts (NICS), and atoms in molecules (AIM) analyses. The structure of isomer 1a is planar with C2v symmetry, whereas its neutral counterpart 1n is non-planar with C2 symmetry, in which its terminal aluminum atoms are out of the plane. The central allenic carbon atom of isomers 1a and 1n exhibits tri-coordination and thus makes it a case of five bonds to carbon, which is confirmed through their total bond order as observed in WBI. Both the isomers show σ- and π-aromaticity and are predicted with the NICS and AdNDP analyses. Further, the results of ab initio molecular dynamics simulations reveal their kinetic stability at room temperature; thus, they are experimentally viable systems.