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.