Experimental and theoretical investigations of CB₈⁻: towards rational design of hypercoordinated planar chemical species

We demonstrated in our joint photoelectron spectroscopic and ab initio study that wheel-type structures with a boron ring are not appropriate for designing planar molecules with a hypercoordinate central carbon based on the example of CB₈, and CB₈⁻ clusters. We presented a chemical bonding model, derived from the adaptive natural density partitioning analysis, capable of rationalizing and predicting planar structures either with a boron ring or with a carbon atom occupying the central hypercoordinate position. According to our chemical bonding model, in the wheel-type structures the central atom is involved in delocalized bonding, while peripheral atoms are involved in both delocalized bonding and two-center two-electron (2c–2e) σ-bonding. Since carbon is more electronegative than boron it favors peripheral positions where it can participate in 2c–2e σ-bonding. To design a chemical species with a central hypercoordinate carbon atom, one should consider electropositive ligands, which would have lone pairs instead of 2c–2e peripheral bonds. Using our extensive chemical bonding model that considers both σ- and π-bonding we also discuss why the AlB₉ and FeB₉⁻ species with octacoordinate Al and Fe are the global minima or low-lying isomers, as well as why carbon atom fits well into the central cavity of CAl₄²⁻ and CAl₅⁺. This represents the first step toward rational design of nano- and subnano-structures with tailored properties.