Structure and Bonding of KSiH₃ and Its 18-Crown-6 Derivatives: Unusual Ambidentate Behavior of the SiH₃^– Anion

Density functional theory (DFT) calculations of [K(18-crown-6)SiH3] (1) and KSiH3 (2) have shown that both the classical tet and non-classical inv coordination modes of the [SiH3]− anion to the K+ ion are energetically accessible. Single-crystal X-ray structures of the tet and inv derivatives [K(18-crown-6)SiH3·THF] (1a) and [K(18-crown-6)SiH3·HSiPh3] (1b) confirm this conclusion, showing that small changes in the coordination sphere of the metal are sufficient to alter the orientation of the anion. A topological analysis of the calculated electron densities for 1 and 2 reveals that the K···Si interaction in the tet conformer of 2 possesses a significant amount of covalent character. In contrast, the inv form of 2 displays primarily electrostatic character for the K···Si and K···H interactions. Incorporation of the 18-crown-6 ligand in 1 reduces the polarizing power of the K+ cation, hardening the cation–anion interaction in both conformers. The experimental structures of 1a and 1b bear out these conclusions, with the strongly bound tetrahydrofuran (THF) ligand softening the K+ ion in 1a and favoring the tet conformer, while the weakly interacting HSiPh3 ligand in 1b has minimal effect on the K+ center, resulting in an inv orientation.对[K(18-冠-6)SiH₃]（化合物1）与KSiH₃（化合物2）开展的密度泛函理论（Density functional theory, DFT）计算结果表明，[SiH₃]⁻阴离子与K⁺阳离子之间的经典四面体（tet）配位模式与非经典反转式（inv）配位模式均在能量上可及。单晶X射线衍射结构表征显示，四面体配位模式衍生物[K(18-冠-6)SiH₃·四氢呋喃(THF)]（1a）与反转式配位模式衍生物[K(18-冠-6)SiH₃·HSiPh₃]（1b）验证了上述结论，即金属配位球的微小变化即可改变阴离子的配位取向。对化合物1和2的计算电子密度开展拓扑分析后发现，化合物2的四面体构象中K···Si相互作用具有显著的共价性特征；与之相反，化合物2的反转式构象中K···Si与K···H相互作用则主要表现为静电相互作用特征。在化合物1中引入18-冠-6配体，可降低K⁺阳离子的极化能力，使两种构象中的阳离子-阴离子相互作用更趋刚性。化合物1a和1b的实验结构进一步验证了这一结论：1a中强结合的四氢呋喃（THF）配体弱化了K⁺阳离子的极化能力，更倾向于形成四面体配位构象；而1b中弱相互作用的HSiPh₃配体对K⁺中心影响极小，最终形成反转式配位取向。