Ab initio valence bond calculations were performed to assess the sigma and pi aromatic character of the all-metallic species Al-4(2-). The results show that the sigma 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 sigma system is significantly higher than that of the pi system (123 vs. 40 kcal/mol), and the pi resonance energy is substantially lower than that of the pi isoelectronic; hydrocarbon C4H42+ (167 kcal/mol). (C) 2004 Elsevier B.V. All rights reserved.