Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50370 Wrocław, Poland.
Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322-0300, USA.
Molecules. 2018 Jun 11;23(6):1416. doi: 10.3390/molecules23061416.
The σ-hole tetrel bonds formed by a tetravalent molecule are compared with those involving a π-hole above the tetrel atom in a trivalent bonding situation. The former are modeled by TH₄, TH₃F, and TH₂F₂ (T = Si, Ge, Sn) and the latter by TH₂=CH₂, THF=CH₂, and TF₂=CH₂, all paired with NH₃ as Lewis base. The latter π-bonded complexes are considerably more strongly bound, despite the near equivalence of the σ and π-hole intensities. The larger binding energies of the π-dimers are attributed to greater electrostatic attraction and orbital interaction. Each progressive replacement of H by F increases the strength of the tetrel bond, whether σ or π. The magnitudes of the maxima of the molecular electrostatic potential in the two types of systems are not good indicators of either the interaction energy or even the full Coulombic energy. The geometry of the Lewis acid is significantly distorted by the formation of the dimer, more so in the case of the σ-bonded complexes, and this deformation intensifies the σ and π holes.
四价分子形成的 σ-hole 键与三价键合情况下位于四价原子上方的π-hole 形成的键进行了比较。前者由 TH₄、TH₃F 和 TH₂F₂（T = Si、Ge、Sn）模拟，后者由 TH₂=CH₂、THF=CH₂ 和 TF₂=CH₂与 NH₃ 作为路易斯碱配对。尽管 σ 和 π-hole 强度接近相等，但后者的π键合配合物的结合强度要强得多。π-二聚体具有更大的结合能归因于更大的静电引力和轨道相互作用。每一次用 F 取代 H，无论是 σ 还是 π，都会增加四键的强度。两种体系中分子静电势最大值的大小都不能很好地指示相互作用能，甚至不能完全指示库仑能。路易斯酸的几何形状由于二聚体的形成而发生显著变形，在 σ 键合配合物的情况下更为明显，这种变形加剧了 σ 和 π 空穴。
