The Electronic Structure of the Al3− Anion: Is it Aromatic?

Multiconfigurationalh igh-level electronic struc-ture calculations show that the Al À 3 ring-likec lustera nion has three close low-lying electronic states of different spin, all of them having strong multiconfigurationalc har-acter.T he aromaticity of the clusterh as, therefore, been studied by meanso ft otal electron delocalization and nor-malized multicenter electron delocalization indicese valu-ated from the multiconfigurationalw ave functions of each state. The lowest-lying singlet and triplet states are found to be highlya romatic, whereas the next lowest-lying state, the quintet state, has much less, though non-negli-gible, aromatic character. The aromaticity of the all-metal clusters has emerged as an ew paradigm of modern chemistry since the seminal discovery by Wang, Boldyrev,a nd co-workers [1] of the [Al 4 X] À (X = Li, Na,C u) cluster family,w hich was rationalized as consisting of the X