Electronic nature of zwitterionic alkali metal methanides, silanides and germanides - a combined experimental and computational approach

Zwitterionic group 14 complexes of the alkali metals of formula [C(SiMe₂OCH₂CH₂OMe)₃M], (M-1), [Si(SiMe₂OCH₂CH₂OMe)₃M], (M-2), [Ge(SiMe₂OCH₂CH₂OMe)₃M], (M-3), where M = Li, Na or K, have been prepared, structurally characterized and their electronic nature was investigated by computational methods. Zwitterions M-2 and M-3 were synthesized via reactions of [Si(SiMe₂OCH₂CH₂OMe)₄] (2) and [Ge(SiMe₂OCH₂CH₂OMe)₄] (3) with MOBu^t (M = Li, Na or K), resp., in almost quantitative yields, while M-1 were prepared from deprotonation of [HC(SiMe₂OCH₂CH₂OMe)₃] (1) with LiBu^t, NaCH₂Ph and KCH₂Ph, resp. X-ray crystallographic studies and DFT calculations in the gas-phase, including calculations of the NPA charges confirm the zwitterionic nature of these compounds, with the alkali metal cations being rigidly locked and charge separated from the anion by the internal OCH₂CH₂OMe donor groups. Natural bond orbital (NBO) analysis and the second order perturbation theory analysis of the NBOs reveal significant hyperconjugative interactions in M-1-M-3, primarily between the lone pair and the antibonding Si-O orbitals, the extent of which decreases in the order M-1 > M-2 > M-3. The experimental basicities and the calculated gas-phase basicities of M-1-M-3 reveal the zwitterionic alkali metal methanides M-1 to be significantly stronger bases than the analogous silanides M-2 and germanium M-3.