A ternary Li₃B₇Si₃ cluster: when reduced electron delocalization enhances thermodynamic stability

The stability and electronic structure of B₇Si₃ ^q (q = +, 0, -, 2- and 3-), Li₃B₇Si₃ and Li₃B₁₀H₃ clusters have been investigated through a comprehensive isomer search, with a focus on the perfect triangular planar isomer T⁺ of B₇Si₃ ⁺. Despite its double aromaticity, this form is thermodynamically unstable. Two strategies for the stabilization of the triangular form are proposed: (i) sequential addition of σ electrons and (ii) incorporation of Li atoms to reduce excessive negative charge. The former reveals that the addition of up to four electrons gradually improves its stability, with T^2- being the most favourable isomer in this charged state. The latter strategy leads to the star form emerging as a global minimum of the ternary Li₃B₇Si₃ cluster, which is significantly more stable. To elucidate the origin of these stabilizations, ring current maps and AdNDP, ELF_σ and ETS-NOCV analyses were employed. Results show a shift from the dominant σ aromaticity in T⁺ to a more balanced σ-π aromatic character in Li₃B₇Si₃, akin to benzene. The AdNDP analysis reveals that T⁺ possesses four σ delocalized bonds, whereas Li₃B₇Si₃ contains only three, serving as one of the indicators of reduced σ-delocalization. ELF_σ analysis further revealed an enhanced peripheral σ delocalization in Li₃B₇Si₃, highlighting the critical role of edge-localized electrons in stabilizing planar aromatic clusters.