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 [XnM3](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 sigma-character. Interestingly, the interaction of these small clusters with alkali metals stabilizes the cyclo-[M-3](2-) ring and leads to a change from sigma-aromaticity in the bound state of the cyclo-[M-3](2-) to pi-aromaticity in the XM3- and X2M3 metallic clusters. Our results also show that the aromaticity of the cyclo-[M-3](2-) unit in the X2M3 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 Na2Mg3, Li2Mg3, and X2Ca3 clusters undergo drastic aromaticity alterations when changing the distance from X to the center of the M-3 ring, whereas X2Be3 and K2Mg3 keep its aromaticity relatively constant along this process. (C) 2009 Wiley Periodicals, Inc. J Comput Chem 30: 2764-2776, 2009