Electron-Rich Bonding and the Importance of s,p Mixing as One Moves Across a Period: A Lesson from the LiSn System

The electronic structure of an unusual LiSn phase (computed using band structure calculations in the framework of the extended Hu¨ckel tight binding theory) is the starting point for a general analysis of the variation of electron-rich multicenter bonding across a period. The LiSn crystal structure of Mu¨ller and Scha¨fer in question contains 2D slabs of Sn atoms arranged as microscopic stairs and intercalatedby Li atoms. Discrepancies between an electron count derived from a recent extension of the Zintl-Klemm rules to electronrich systems (52/3 electrons) and the experimental one (5 electrons for the Sn sublattice) and other failures of chemical common sense emerge in the analysis. The key for interpretation of a series of puzzling results was found in the comparative analysis of the Sn net with other main group element hypervalent slabs. Increasing s,p-mixing as one moves from the right to the left side of the same row of the periodic table is responsible for these effects. The result is that a lower electron count is found in the Sn slabs relative to the one expected from the extended Zintl-Klemm theory. The effect should also occur in discrete molecules. We also showed that the Li atoms have a role in the determination of the final structure, not only because of their small size but also through the degree of the electron transfer to the Sn sublattice.