Unusual Structures of Dilithiosilanes and Disodiosilanes: Ionicity of the Silicon-Alkali-Metal Bond

Both dilithiosilanes (SiH₂Li₂) and disodiosilanes (SiH₂Na₂) favor unusual structures. The lowest singlet and lowest triplet potential energy surfaces (PES) were searched with the 3-21G and 3-21G(*) basis sets, and a number of stationary points were located. The “normal” tetrahedral-based singlet, 3s (C_2v), was the lowest energy structure at HF but not at correlated levels of theory. The most interesting singlet SiH₂Li₂structures were reoptimized at HF/6-31G, and the final energies were calculated at MP4SDTQ/6-31G and corrected for the zero-point vibrational energy. in the global minimum structure (1s), the lithiums are unsymmetrically placed in the C_splane orthogonal to the SiH₂moiety. This form is 6.3 kcal mol^-1lower in energy than the conventional tetrahedral singlet (3s) and 3.9 kcal mol^-1lower than the inverted tetrahedral singlet (2s). The³B₂state with a conventional tetrahedral geometry, lying only 14.1 kcal mol^-1above 1s at UMP2/6-31G*//3-21G(*) + ZPVE, is the lowest triplet 1t. Other local triplet minima include 2t (³A“, C_s) with all atoms in the same plane, 3t (³B₂) with an inverted tetrahedral structure, 4t (³A') with a structure similar to 1s, and 5t (³B₁C_2v) with collinear SiLiLi. The triplets 2t, 3t, 4t, and 5t are 15.0, 18.0, 20.8, and 25.8 kcal mol^-1, respectively, above the global singlet minimum. Some of the corresponding energy minima were calculated for disodiosilanes at 3-21G. The ionicity of the SiLi and SiNa bonds is compared to that of the CLi and CNa bonds by means of NPA and integrated projected electron population analyses.