Structure and Bonding in CE₅⁻ (E=Al–Tl) Clusters: Planar Tetracoordinate Carbon versus Pentacoordinate Carbon

Abstract

The structure, bonding, and stability of clusters with the empirical formula CE₅^- (E=Al-Tl) have been analyzed by means of high-level computations. The results indicate that, whereas aluminum and gallium clusters have C_2v structures with a planar tetracoordinate carbon (ptC), their heavier homologues prefer three-dimensional C_4v forms with a pentacoordinate carbon center over the ptC one. The reason for such a preference is a delicate balance between the interaction energy of the fifth E atom with CE₄ and the distortion energy. Moreover, bonding analysis shows that the ptC systems can be better described as CE₄^- , with 17-valence electrons interacting with E. The ptC core in these systems exhibits double aromatic (both σ and π) behavior, but the σ contribution is dominating.