Y vs. cyclic delocalization in small ring dications and dianions: The dominance of charge repulsion over Huckel aromaticity

Aromaticity of Group 14 Organometallics: Experimental Aspects

“…Two nonplanar minimum structures of C s and C 2 symmetry with equal energies were found on the PES, whereas the planar delocalized D 5 h structure was 8.3 kcal mol −1 higher in energy. The value of ASE for C s ‐symmetric [Si 5 H 5 ] − was calculated from the isodesmic equation as 52.8 kcal mol −1 , which was much smaller than the 84.7 kcal mol −1 calculated for its carbon analogue [C 5 H 5 ] − , thus pointing to a significantly lower extent of aromaticity 25. Nevertheless, this value was still much greater than that for the silacyclopentadienide ion [C 4 H 4 SiH] − (2.2 kcal mol −1 ), which was calculated on the basis of an analogous isodesmic reaction 48.…”

“…As was demonstrated in Section 2.2, the behavior of the small doubly positively charged systems, such as the cyclobutadiene dication with two π electrons, is greatly dictated by the severe Coulomb repulsive interaction between the two charges, which might overcome the tendency toward π delocalization and aromatic stabilization 21a. 24, 25 The same trends evidently operate in the case of the doubly negatively charged cyclobutadiene dianion with six π electrons (Scheme ; E=C), whose story is strikingly different from that of its singly negatively charged analogue with six π electrons, the cyclopentadienide ion (see Section 3.2).…”

“…21a, 22, 23 Consequently, it was concluded that in small doubly charged four‐membered ring systems, such as the cyclobutadiene dication and cyclobutadiene dianion, the Coulomb repulsive interaction between the two charges becomes more important than the advantage gained by the π delocalization and therefore may overcome the effects of Hückel aromatic stabilization 21a. 24, 25 The latest computations by Schleyer et al. corroborate well with the previous conclusions, confirming the preference of the folded structure for the cyclobutadiene dication and its alkyl derivatives (Scheme ; E=C, R=H, alkyl), which were nevertheless aromatic according to NICS(0) calculations: −9.0 (unsubstituted cyclobutadiene dication, D 2 d minimum structure), −17.2 (tetra‐ tert ‐butylcyclobutadiene dication, C 2 minimum structure) 26…”

“…An particularly exciting analogue of the cyclopentadienide ion, the pentasilacyclopentadienide ion [Si 5 H 5 ] − , whose structure consists exclusively of heavier Group 14 elements, was computationally studied by Schleyer and co‐workers 25. Two nonplanar minimum structures of C s and C 2 symmetry with equal energies were found on the PES, whereas the planar delocalized D 5 h structure was 8.3 kcal mol −1 higher in energy.…”

Aromatizität von metallorganischen Verbindungen mit Gruppe‐14‐Elementen: experimentelle Aspekte

“…21a, 22, 23 Somit gelangte man zu dem Schluss, dass in doppelt geladenen Vierringsystemen wie dem Cyclobutadien‐Dikation oder ‐Dianion die Coulomb‐Abstoßung zwischen den beiden Ladungen den Vorteil, der aus der π‐Delokalisierung resultiert, überwiegt und so die Effekte der aromatischen Hückel‐Stabilisierung aufhebt 21a. 24, 25 Die neuesten Rechnungen von Schleyer et al. stützen die bisherigen Schlussfolgerungen und bestätigen für das Cyclobutadien‐Dikation und dessen Alkyl‐Derivate die Bevorzugung gefalteter Strukturen (Schema : E=C, R=H, Alkyl), die aber NICS(0)‐Rechnungen zufolge trotzdem aromatisch sind: −9.0 (unsubstituiertes Cyclobutadien‐Dikation, D 2 d ‐Minimumstruktur), −17.2 (Tetra‐ tert ‐butylcyclobutadien‐Dikation, C 2 ‐Minimumstruktur) 26…”