Tuning aromaticity in trigonal alkaline earth metal clusters and their alkali metal salts

In this work, we analyze the geometry and electronic structure of the [X n M 3 ] n −2 species (M = Be, Mg, and Ca; X = Li, Na, and K; n = 0, 1, and 2), with special emphasis on the electron delocalization properties and aromaticity of the cyclo ‐[M 3 ] 2− unit. The cyclo ‐[M 3 ] 2− ring is held together through a three‐center two‐electron bond of σ‐character. Interestingly, the interaction of these small clusters with alkali metals stabilizes the cyclo ‐[M 3 ] 2− ring and leads to a change from σ‐aromaticity in the bound state of the cyclo ‐[M 3 ] 2− to π‐aromaticity in the XM 3 − and X 2 M 3 metallic clusters. Our results also show that the aromaticity of the cyclo ‐[M 3 ] 2− unit in the X 2 M 3 metallic clusters depends on the nature of X and M. Moreover, we explored the possibility for tuning the aromaticity by simply moving X perpendicularly to the center of the M 3 ring. The Na 2 Mg 3 , Li 2 Mg 3 , and X 2 Ca 3 clusters undergo drastic aromaticity alterations when changing the distance from X to the center of the M 3 ring, whereas X 2 Be 3 and K 2 Mg 3 keep its aromaticity relatively constant along this process. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009