Substituent effects on stability, MEP, NBO analysis, and reactivity of 2,2,9,9-tetrahalosilacyclonona-3,5,7-trienylidenes, at density functional theory.

Cyclonona-3,5,7-trienylidene appears as boat-shaped transition state for having a negative force constant, while its singlet state exhibits less stability than the corresponding triplet state. Succeeding the quest for the largest unsaturated stable carbene-like species, theoretical investigations coupled with suitable isodesmic reactions are used to examine the effects of α,αʹ-tetrahalo groups on the thermodynamic along with kinetic viabilities of nine-membered cyclic silylenes. All the singlet and triplet silylenes appear as boat-shaped minima for having positive force constants on their potential energy surfaces and singlet states emerge as ground state, exhibiting more stability than their corresponding triplet states. The order of stability estimated by singlet (S)–triplet (T) energy separation (ΔE_S–T = E_{T −} E_S) emerges as α,αʹ-tetrahydrocarbene < α,αʹ-tetrahydrosilylene < α,αʹ-tetrafluorosilylene < α,αʹ-tetraiodosilylene < α,αʹ-tetrachlorosilylene < α,αʹ-tetrabromosilylene. This research specifies band gap (ΔE_HOMO–LUMO) of scrutinized silylenes with this order. Hence, singlet 2,2,9,9-tetrabromosilacyclonona-3,5,7-trienylidene exists as the most stable species. From both thermodynamic and kinetic points of view, this species is more stable than synthesized silylene by Kira. It shows the highest heat of dehydrogenation through isodesmic reaction. The NBO analysis provides significant evidences for the stability of it through positive hyperconjugation, negative hyperconjugation, as well as mesomeric effects. [ABSTRACT FROM AUTHOR]