CB5Pd5+: a boron-based global minimum with a planar pentacoordinate carbon

Abstract

Although boron has been widely employed in constructing molecular systems containing a non-classical planar hypercoordinate carbon (phC) center, its inherent tendency towards multicenter bonding creates significant challenges in stabilizing boron-based phC configurations. Notably, nearly all boron-based phC species are energetically less stable than their isomers with a boron atom located at the phC site. Herein, we report the successful design of a novel boron-based planar pentacoordinate carbon (ppC) cluster, CB₅Pd₅⁺, through incorporation of palladium bridging units. Remarkably, this ppC configuration demonstrates superior thermodynamic stability, being 45.1 kcal mol⁻¹ lower in energy than that of its planar pentacoordinate boron (ppB) isomer. Electronic structure analysis shows that carbon atom can occupy the central position of the cluster owing to the fact that five bridging palladium atoms eliminate the electron-deficient character of boron atoms through the Pd→B back-bonding. This finding demonstrates that electron compensation mechanisms in hypercoordinate systems are not exclusive to conventional chalcogens (such as S, O, or Se atoms), and that transition metal elements can also show comparable effects.