First synthesis and structural determination of a monomeric, unsolvated lithium amide, LiNH₂

Alkali metal amides typically aggregate in solution and the solid phase, and even in the gas phase. In addition, even in the few known monomeric structures, the coordination number of the alkali metal is raised by binding of Lewis-basic solvent molecules, with concomitant change in structure. In contrast, the simplest lithium amide LiNH₂ has never been made in a monomeric form, even though its structure has been theoretically predicted several times. Here, the first experimental structural data for a monomeric, unsolvated lithium amide are determined using a combination of gas-phase synthesis and millimeter/submillimeter-wave spectroscopy. All data point to a planar structure for LiNH₂. The r_o structure of LiNH₂ has a Li-N distance of 1.736(3) Å, an N-H distance of 1.022(3) Å, and a H-N-H angle of 106.9(1)°. These results are compared with theoretical predictions for LiNH₂, and experimental data for oligomeric, solid-phase samples, which could not resolve the question of whether LiNH₂ is planar or not. In addition, comparisons are made with revised gas-phase and solid-phase data and calculated structures of NaNH₂.