Structural Characterization of a Boron(III) η²‑σ-Silane-Complex

Coordination of H–E (E = H, C, Si, etc.) σ-bonds to unsaturated element centers, forming η2-σ-complexes, is a crucial step in H–E bond activations which are involved in numerous chemical transformations. Structural characterization of σ-complexes has provided invaluable insight into the σ-bond activation process. While numerous η2-σ-complexes of low-oxidation-state transition metals have been isolated and thoroughly studied, those based on d0 metals or high-oxidation-state main group elements still remain elusive, despite their suggested role as key intermediates in σ-bond metathesis. Here we report the synthesis of a highly electrophilic o-carboranyl-substituted hydroborenium and its η2-σ-(H–Si)-complex, the latter of which was characterized by X-ray analysis. Unlike previous known σ-(H–Si)-complexes in which back-donation contributes significantly to their stability, our computational studies revealed that no back-donation takes place between the borenium and silane moieties, and the key factors for the stability of this silane-borenium complex are (1) the strong interaction between the empty p orbital of the boron center and the bonding σ orbital of H–Si and (2) steric accessibility of the boron center due to the presence of a hydrogen substituent