QUANTUM CHEMICAL INVESTIGATION OF SILYL CATION–LEWIS BASE COMPLEXES: STRUCTURAL FEATURES AND STABILITY VIA DFT AND NBO ANALYSIS

Abstract

This work delves into how steric and electronic influences govern the structural and energetic properties of Si–P bonds within silicon-based cationic systems coordinated to phosphine ligands. Through comprehensive computational analysis, it is shown that (CH₃)₃Si⁺ forms the most compact Si–P linkages (2.01–2.03 Å), a result of low spatial hindrance and effective orbital overlap. Slightly elongated bond lengths (2.03–2.05 Å) occur with (C₂H₅)₃Si⁺, while the most extended distances (2.08–2.10 Å) are associated with Ar₃Si⁺, where both steric congestion and electron-withdrawing aromatic rings impede close approach. The angular parameters reflect similar trends; bulky phosphines such as (C₂H₅)₃P induce broader Si–P–R bond angles (e.g., up to 121° with Ar₃Si⁺) relative to less hindered analogs like (CH₃)₃P.

Energetically, the (CH₃)₃Si⁺···(CH₃)₃P pair exhibits the strongest interaction (~–28 kcal/mol), a result of well-balanced spatial arrangement and electrostatic compatibility. In contrast, electron donation trends derived from charge transfer calculations reveal that (C₂H₅)₃P is more effective in transferring electron density than (CH₃)₃P, aligning with predictions based on steric and electronic parameters. Natural Bond Orbital (NBO) evaluations further demonstrate that increased σ-donation correlates with greater interaction strength, as evidenced by more negative interaction energies. Conversely, Ar₃Si⁺ consistently engages in weaker associations, hindered by the electron-withdrawing character of its aromatic substituents.

Altogether, this study emphasizes the nuanced interdependence between geometry and electronic structure in modulating Si–P bonding, offering design principles for future silicon-phosphine architectures relevant to catalysis and materials development.