Influence of monomer deformation on the competition between two types of σ-holes in tetrel bonds

One of several tetrel (T) atoms was covalently attached to three F atoms and a substituted phenyl ring. A NH 3 base can form a tetrel bond with TF 3 C 6 H 2 R 3 (T = Si, Ge, Sn, Pb; R = H, F, CH 3 ) in a position opposite either an F atom or the ring. The σ-hole opposite the highly electron-withdrawing F (T-F) is more intense than that opposite the ring (T-C). However, when the Lewis base deforms from a tetrahedral to a trigonal bipyramidal shape so as to accommodate the base, it is the T-C σ-hole that is more intense. Accordingly, it is the T-C tetrel-bonded complex for which there is a larger interaction energy with NH 3 , as high as 34 kcal mol −1 . Countering this trend, it requires more energy for the TF 3 C 6 H 2 R 3 to deform into the geometry it adopts within the T-C complex than within its T-F counterpart. There is consequently a balance between the overall binding energies of the two competing sites. The smaller tetrel atoms Si and Ge, with their larger deformation energies, show a preference for T-F tetrel binding, while the T-C site is preferred by Pb which suffers from a smaller degree of deformation energy. There is a near balance for T = Sn and the two sites show comparable binding energies. Competition between two competing sites on a tetrel atom is explained by balance between structural deformation and σ-hole intensity.