Boron‐Based Chiral Helix Be6B102− and Be6B11− Clusters: Structures,Chemical Bonding,and Formation Mechanism

Boron forms a rich variety of low‐dimensional nanosystems, including the newly discovered helix Be₆B₁₀^2? ( 1 ) and Be₆B₁₁^? ( 2 ) clusters. We report herein on the elucidation of chemical bonding in clusters 1 / 2 , using the modern quantum chemistry tools of canonical molecular orbital analyses and adaptive natural density partitioning (AdNDP). It is shown that clusters 1 / 2 contain a chiral helix Be₂B₁₀Be₂ or Be₂B₁₁Be₂ skeleton with a total of 11 and 12 segments, respectively, which effectively curve into “helical pseudo rings” and chemically consist of two “quasicircles” as defined by their anchoring Be centers. The helix skeleton is connected via Lewis‐type B?B and Be?B?Be σ bonds, being further stabilized by island π/σ bonds and a loose π bond at the junction. The Be₆ component in 1 / 2 assumes a distorted prism shape only physically, and it is fragmented into four parts: two terminal Be₂ dimers and two isolated Be centers. A Be₂ dimer at the far end manages to bend over and cap a quasicircle from one side of B plane. Consequently, each quasicircle of a helical pseudo ring is capped from opposite sides by two Be₂/Be units, facilitating intramolecular charge‐transfers of 5 electrons from Be to B. Overall, the folding of B helix involves as many as 10 electrons. The enormous electrostatics offers the ultimate driving forces for B helix formation.