Strain in nonclassical silicon hydrides: An insight into the “ultrastability” of sila‐bi[6]prismane (Si 18 H 12 ) cluster with the endohedrally trapped silicon atom, Si 19 H 12

The recently postulated concept of “ultrastability” and “electron‐deficient aromaticity” (Vach, Nano Lett 2011, 11, 5477; Vach, J Chem Theory Comput 2012, 8, 2088) in a sila‐bi[6]prismane having an additional entrapped silicon atom, Si 19 H 12 , has been disproved on the basis of a careful analysis of the energetic characteristics related to the formation of this and other silicon hydrides. The central silicon atom in Si 19 H 12 is weaker bound to other silicon atoms than in conventional tetrahedral silanes; moreover, Si 19 H 12 possesses a significant amount of strain. The role of strain in the formation of the title compounds has been further rationalized by calculating the relative energies for the transformation to a half‐planar conformation in methane and in silane and by calculating the respective strain energies. The strain energy value in Si 18 H 12 is equal to 9.93 eV whereas the same property for Si 19 H 12 lies in range of 6.42–8.85 eV. Two low‐energy isomers of Si 19 H 12 which lie by 2.77 and 3.42 eV (!) lower in energy than the originally considered sila‐bi[6]prismane‐based structure have been proposed. © 2015 Wiley Periodicals, Inc.