Al₄(P^<i>t</i>Bu₂)₆ − a Derivative of Al₄H₆ − and Other Al₄ Species: A Challenge for Bonding Interpretation between Zintl Ions and Metalloid Clusters

The highly energetic molecule Al4H6, with its distorted tetrahedral structure, was recently characterized via mass spectrometry and photoelectron spectroscopy investigations (Li, X.; et al. Science 2007, 315, 356). Here we present the preparation and structural investigation of the first analogous Al4R6 cluster compound. In order to understand the bonding in this kind of Al4 molecule, density functional theory and second-order Møller−Plesset perturbation theory calculations were performed. The results obtained are discussed in comparison with bonding in other Al4 moieties, especially the aromatic bonding behavior in the dianionic planar Al42− species (Li, X.; et al. Science 2001, 291, 859). Finally, on the basis of the results obtained for Al4 species, a more general problem is discussed: the difference in bonding between Zintl ions and metalloid clusters.