Inter/Intramolecular Bonds in TH5+ (T = C/Si/Ge): H2 as Tetrel Bond Acceptor and the Uniqueness of Carbon Bonds.

Abstract

Atoms in molecules (AIM), natural bond orbital (NBO), and normal coordinate analysis have been carried out at the global minimum structures of TH5+ (T = C/Si/Ge). All these analyses lead to a consistent structure for these three protonated TH4 molecules. The CH5+ has a structure with three short and two long C-H covalent bonds and no H-H bond. Hence, the popular characterization of protonated methane as a weakly bound CH3+ and H2 is inconsistent with these results. However, SiH5+ and GeH5+ are both indeed a complex formed between TH3+ and H2 stabilized by a tetrel bond, with the H2 being the tetrel bond acceptor. The three-center-two-electron bond (3c-2e) in CH5+ has an open structure, which can be characterized as a V-type 3c-2e bond and that found in SiH5+ and GeH5+ is a T-type 3c-2e bond. This difference could be understood based on the typical C-H, Si-H, Ge-H, and H-H bond and the tetrel bond energies. This analysis explains the trend observed in proton affinity of TH4 which appears counterintuitive, GeH4 > SiH4 > CH4. Carbon is selective in forming a "tetrel bond" and when it does, it might be worthwhile to highlight it as a "carbon bond".