A valence bond study of the σ and π aromatic species Al42−

Ab initio valence bond calculations were performed to assess the σ and π aromatic character of the all-metallic species Al42−. The results show that the σ system is composed from two independent systems (built from the radial and tangential p-orbitals), each containing two delocalised electrons, providing the conduction. The resonance energy of the σ system is significantly higher than that of the π system (123 vs. 40 kcal/mol), and the π resonance energy is substantially lower than that of the π isoelectronic hydrocarbon C4H42+ (167 kcal/mol).