Multiple aromaticity and antiaromaticity in silicon clusters

A series of silicon clusters containing four atoms but with different charge states (Si-4(2+), Si-4, Si-4(2-), and NaSi4-) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change Si,21 upon addition or removal of two electrons. It is shown that is square-planar, analogous to the recently discovered aromatic Al-4(2-) cluster. Upon addition of two electrons, neutral Si-4 becomes sigma-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si-4 leads to two energetically close structures of Si-4(2-): either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si-4(2-), a stabilizing cation (Na+) was used to produce Si-4(2-) in the gas phase in the form of No+[Si-4(2-)], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+[Si-4(2-)] species is highly unusual.