Theoretical study on the aromaticity from d-AOs in cationic X(3)(+) (X = Sc, Y, La) clusters

The stable structures and aromatic characters for three cationic X(3)(+) (X = Sc, Y, and La) and three relevant neutral X(3)Cl (X = Sc, Y, La) clusters are investigated at the DFT and post HF level of theory. The calculated results show that the X(3)(+) cations each has two stable structures: the regular trigon (D(3h)) and the line (D∞h) with the regular trigon (D(3h)) being the ground state, while for three neutral X(3)Cl clusters, Sc(3)Cl has three stable isomers: the trigon-pyramidal (C(3v)), bidentate (C(2v)-1), and C(2v)-2 structures, Y(3)Cl and La(3)Cl each has only two stable isomers: the trigon-pyramidal (C(3v)) and bidentate (C(2v)-1) structures. The ground states for three X(3)Cl species are all the bidentate (C(2v)-1) isomers. The calculations of the resonance energy (RE) and NICS show that trigonal X(3)(+) isomers exhibit higher degree of aromaticity. The detailed molecular orbital analyzes reveal that the isolated trigonal Sc(3)(+) and Y(3)(+) cations each has one delocalized π-type MO and shows single π-aromaticity, while the isolated trigonal La(3)(+) cation has one delocalized σ-type MO and shows single σ-aromaticity. The single π- or σ-aromaticity for X(3)(+) are attributed to the contributions mainly from the d AOs of the corresponding transition metal X atoms. However, when a singly negatively charged counterion Cl(-) is added to Sc(3)(+), Y(3)(+), and La(3)(+) cations respectively, the aromatic type for the two Sc(3)(+), Y(3)(+) units in the corresponding neutral Sc(3)Cl, Y(3)Cl complexes are changed from π-aromaticity into σ-aromaticity, whereas the σ-aromaticity of the La(3)(+) units in the La(3)Cl complex keeps unchanged in this process. Thus three Sc(3)(+), Y(3)(+), La(3)(+) units in the corresponding X(3)Cl complexes all have only one σ-type MO and exhibit single σ-aromaticity.