Abstract
When an N-base approaches the tetrel atom of TF4 (T = Si, Ge, Sn) the latter molecule deforms from a tetrahedral structure in the monomer to a trigonal bipyramid. The base can situate itself at either an axial or equatorial position, leading to two different equilibrium geometries. The interaction energies are considerably larger for the equatorial structures, up around 50 kcal/mol, which also have a shorter R(T··N) separation. On the other hand, the energy needed to deform the tetrahedral monomer into the equatorial structure is much higher than the equivalent deformation energy in the axial dimer. When these two opposite trends are combined, it is the axial geometry which is somewhat more stable than the equatorial, yielding binding energies in the 8–34 kcal/mol range. There is a clear trend of increasing interaction energy as the tetrel atom grows larger: Si < Ge < Sn, a pattern which is accentuated for the binding energies.
Highlights
The continuing development of modern chemistry is leading to improved understanding of microscale processes
Within the context of a tetrahedral geometry there is a negative region surrounding each of the F atoms, and a positive area that lies directly opposite each T–F bond, known generally as a σ-hole
The molecular electrostatic potential (MEP) of the four Lewis base derivatives of pyridine are presented in Figure 1, and all contain a negative region that is coincident with the N lone pair direction
Summary
The continuing development of modern chemistry is leading to improved understanding of microscale processes. With its positive electrostatic nature, this hole can attract a nucleophile [14,16,17], and the entire interaction is supplemented by polarization and dispersion forces This idea is not limited to halogens, but has been extended to other electronegative atoms [18,19,20] and has recently expanded to transition metal interactions with Lewis bases, sometimes referred to as a “regium bond” [21]. It is the plane of the molecule which can house a positive region. When this phenomenon occurs above the plane of a molecule, it is commonly dubbed a π-hole [22,23,24]
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