Chapter 10 Electronic structure and reactivity of aromatic metal clusters

It has been argued that some small newly discovered all-metal clusters may be consideredas aromatic. This would make them the first aromatic chemical systems with no carbon content at all. The initial report was on the ionic aluminum cluster, Al₄^2-, which exhibits a planar square structure and has two delocalized π-electrons, thus satisfying Hückel's rule for aromatic systems. It also shows a relatively high chemical and structural stability. By means of quantum chemical calculations, Kuznetsov et al. have shown that the square structure is preserved when the Al cluster reacts to form some bimetallic clusters with various chemical compositions: MAl_4- and M₂Al₄ (where M = Li⁺, Na⁺, and Cu⁺) and that the aromatic nature is apparently preserved. In this paper, we report a density functional theory (DFT) study, using the hybrid functional B3LYP and a 6-31+G(d) basis set, which allows us to rationalize and to quantify the aromatic nature of MAl₄^- species and to extend our study to other bimetallic clusters which have not yet been reported. These clusters have the general formula MA1₄ⁿ(M = Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺, Sc³⁺, Al³⁺, B³⁺, Ga³⁺, and Ti⁴⁺) where n = - 1, 0, +1, +2. We also evaluate the stability as a function of charge and discuss different properties to assess the aromaticity of these systems, namely through reactivity indices such as the DFT absolute hardness, nucleus-independent chemical shift (NICS), and delocalization indexes (DIs).