Which triel bond is stronger? TrHX•••H2Y versus TrH2X•••H2Y (Tr = Ga, In; X = F, Cl, Br, I; Y = O, S)

Triel bond, a recent emerging noncovalent interaction, has attracted extensive attention from physical and chemical scientists. Some triel atoms, such as Ga and In atoms, theirs valence, can be bivalent, trivalent, and even tetravalent. What is the influence of the valence of triel atom on the triel-bonding formation? This issue generates our initial research interest. Then, the influence of valence of triel atom on the triel bond has been theoretically investigated in thirty-two complexes composed by Lewis acids TrHX/TrH2X (Tr = Ga, In; X = F, Cl, Br, I) and Lewis base H2Y (Y = O, S) molecules at MP2/aug-cc-pVTZ(PP) theory level. MEP analyses show that both TrHX and TrH2X molecules possess pi-holes on Tr atoms, which can be used as Lewis acids, and interact with Lewis bases to form triel bonds. AIM and Non-covalent interaction (NCI) analyses demonstrate that triel bond exists in all the studied thirty-two complexes. Hydrogen bond coexists with triel bond only in six of these complexes. In the process of triel bond formation, TrH2X molecule has stronger structural distortion than TrHX molecule. And TrH2X molecule can form a stronger triel bond than that of TrHX molecule by analyzing multiple factors that affect triel-bonding formation including substituents, electron donors, and nature of triel atoms. In addition, NBO analyses show that triel bond in TrH2X-H2Y complexes has stronger orbital interaction than that in TrHX-H2Y complexes.