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In order to accommodate the approach of two NCH bases, a tetrahedral TF4 molecule (T=Si,Ge,Sn,Pb) distorts into an octahedral structure in which the two bases can be situated either cis or trans to one another. The square planar geometry of TF4, associated with the trans arrangement of the bases, is higher in energy than its see-saw structure which corresponds to the cis trimer. On the other hand, the square geometry offers an unobstructed path of the bases to the π-holes above and below the tetrel atom so enjoys a higher interaction energy than is the case for the σ-holes approached by the bases in the cis arrangement. When these two effects are combined, the total binding energies are more exothermic for the cis than for the trans complexes. This preference amounts to some 3 kcal/mol for Sn and Pb, but is amplified for the smaller tetrel atoms.
Michalczyk, M., Zierkiewicz, W., Wysokiński, R., Scheiner, S. I. (2019). Hexacoordinated Tetrel-Bonded Complexes between TF4 (T=Si, Ge, Sn, Pb) and NCH: Competition between σ- and π-Holes. ChemPhysChem, 20(7), 959-966. https://onlinelibrary.wiley.com/doi/abs/10.1002/cphc.201900072