page_title: Silylone
section_title: Theoretical Predictions
hierarchical_section_title: Silylone / Theoretical Predictions
caption_attribution_description: English: Generic formula of trisilaallene
context_page_description: Silylones are a class of zero-valent monatomic silicon complexes, characterized as having two lone pairs and two donor-acceptor ligand interactions stabilizing a silicon center. Synthesis of silylones generally involves the use of sterically bulky carbenes to stabilize highly reactive Si centers. For this reason, silylones are sometimes referred to siladicarbenes. To date, silylones have been synthesized with cyclic alkyl amino carbenes and bidentate N-heterocyclic carbenes. They are capable of reactions with a variety of substrates, including chalcogens and carbon dioxide.
context_section_description: The structure of carbene-stabilized silylones were first predicted using theoretical calculations by Gernot Frenking and coworkers in 2009. Their theoretical study of silylones was inspired from the development and synthesis of carbones: an analogous structure containing carbon(0) stabilized by two donor-acceptor ligand interactions. It was also inspired by previous reports of trisilaallene: a silylene complex featuring a bent geometry about the Si-Si-Si center. The unexpected bent trisilaallene bond angle was dissimilar that of carbon allenes (C=C=C) yet like that of NHC-stabilized carbones. Towards the goal of rationalizing this structure and to investigate silylones in general, the authors analyzed the energetics of di-coordinated model complexes of silicon(0) (L₂Si) and carbon(0) (L₂C). More specifically, the analysis was conducted using boron trihydride (BH₃) binding analysis and proton affinity analysis at the BP86/TZVPP level of theory. 
  Compared to analogous model complexes of carbon, the silicon complexes displayed very different characteristics. For example, for two of the examined models, the structure of L₂C(BH₃)₂ could not be energetically minimized whereas it could be for L₂Si(BH₃)₂. Both the silicon and carbon model complexes contained two lone pair orbitals: one with σ-orbital character and one with π-orbital character. However, bonding of one of the model complexes with a single BH₃ occurred at the π-lone pair for the silicon complex and at the σ-lone pair for the carbon complex. As a consequence, the bonding geometries of the resultant complexes differed. High values of the second proton affinity (e.g. PA = 142.9, 129.3, 166.8, and 123.9 kcal/mol) and of the bond dissociation energy (BDE) of the second BH₃ ligand of di-coordinated BH₃ complexes (e.g. 26.2, 47.8, 48.1, and 36.6 kcal/mol) were also found. In conjunction with frontier orbital analysis, the high electron density found at the silicon centers suggested that the ligands bonded as donors than covalently. Therefore, the authors claimed that the model complexes were better described as silylones than silylenes. Since high values of the proton affinity were also found for trisilallene, the previously reported complex was also suggested to be a silylone rather than as a silylene. As a result of this analysis, the authors encouraged their exploration by experimentalists.
caption: General structure of trisilaallene.
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