Aromatic and o-Antiaromatic Species

Let us assess aromaticity in Li3 . First, this cation has a perfect triangular structure with aU bonds being equal. Second, it has two a-electrons and thus it satisfies the 4 -l- 2 rule for = 0. Third, the resonance energy in lij was evaluated to be quite significant of 36kcalmol [100]. The o-resonance energy in the lij cation can be calculated as the energy of the reaction  

The Mj (M = Li, Na, K, Rb, Cs) anions are examples of a-antiaromatic systems with 4 o-electrons [100]. Singlet Mj anions at the symmetry have the la/ le electron configuration and their triangular structures undergo Jahn-Teller distortion. Indeed, Mj anions are linear. Two valence MOs can be localized into two 2c-2e bonds and the linear structure of Mj can be considered as a classical structure. That is similar to the antiaromatic cyclobutadiene, where canonical MOs can be localized into two double and two single carbon-carbon bonds, producing a classical structure. 

The rationalization of the square structure of the Al4 cluster helped advance the concepts of aromaticity to aU-metal systems in 2001 [106, 107]. The initial goal 

Furthermore, the search for the global minimum of the metastable Al4 (it is not stable with respect to an electron detachment) cluster revealed that the planar square structure was indeed the lowest in energy. The AdNDP analysis shows that four canonical MOs of Al4 can be transformed to four lone pairs with one located on every aluminum atom. Three other canonical MOs stay as four-centered bonds. The HOMO is clearly a completely bonding jt-MO. Two electrons on that MO make this cluster jt-aromatic. The HOMO-1 is a completely bonding MO formed by p -radial AOs. Two electrons on that MO make this cluster -aromatic. The HOMO-2 is a completely bonding MO formed by p,-tangential AOs. Two electrons on that MO make this cluster Oj-aromatic. Thus, this is an example of a system with double (a,.-, and Jt-) aromaticity. 

Two years later, the joint photoelectron and theoretical study of the Li3Al4 cluster showed [108] that this cluster contains the approximately rectangular Al4 cluster, which was reported to be an example of the net all-metal antiaromatic 

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