Synergistic and diminutive effects between triel bond and regium bond: Attractive interactions between π‐hole and σ‐hole

Abstract

High quantum chemical calculations have been performed for binary complexes of MCN···ZX3 (M = Cu, Ag, Au; Z = B, Al; X = H, F) and C2H4···AlX3. The strength of triel bonding depends on the nature of triel and coin metal atoms as well as the F substituents and electron donors. The molecular electrostatic potential (MEP) analysis confirms a σ‐hole at the M‐C bond end of MCN, engaging in a regium bond with C2H4 in an increasing sequence of AgCN < CuCN < AuCN. The complex C2(CN)4···AuCN is unstable in view of MEPs, but a big attractive interaction energy (−38 kcal/mol) is produced when both molecules approach, which is mainly caused by polarization including orbital interactions. Both types of interactions are strengthened in ternary complex of C2H4···MCN···ZX3 but are weakened in NCAu···C2H4···AlX3 and C2(CN)4···AuCN···ZH3. It is found that the variation from synergistic to diminutive effects can be modulated by four CN groups in C2(CN)4. Interestingly, the binding distances of both interactions have an unexpected change. The cooperativity of both interactions has been explained with MEP and charge transfer. When C2H4 binds with AlX3 or AuCN, its π electron density is greatly decreased and even its MEP becomes positive, but it is still able to participate in a regium bond or a triel bond.