A Test to Evaluate the Performance of Aromaticity Descriptors in All-Metal and Semimetal Clusters. An Appraisal of Electronic and Magnetic Indicators of Aromaticity

As compared to classical organic aromatic compounds, the evaluation of aromaticity in all-metal and semimetal clusters is much more complex. For a series of these clusters, it is frequently found that different methods used to discuss aromaticity lead to divergent conclusions. For this reason, there is a need to evaluate the reliability of the different descriptors of aromaticity to provide correct trends in all-metal and semimetal aromatic clusters. This work represents the first attempt to assess the performance of aromaticity descriptors in all-metal clusters. To this end, we introduce the series of all-metal and semimetal clusters [XnY4-n](q +/-) (X, Y = Al, Ga, Si, and Ge; n = 0-4) and [XnY5-n](4-n) (X = P and Y = S and Se; n = 0-5) with predictable aromaticity trends. Aromaticity, in these series, is quantified by means of nucleus-independent chemical shifts (NICS) and electronic multicenter indices (MCI). Results show that the expected trends are generally better reproduced by MCI than by NICS. It is found that NICS(0)(pi) is the kind of NICS that performs better among the different NICS indices analyzed.