Comparison of π-hole tetrel bonding with σ-hole halogen bonds in complexes of XCN (X = F, Cl, Br, I) and NH3.

In addition to the standard halogen bond formed when NH3 approaches XCN (X = F, Cl, Br, I) along its molecular axis, a perpendicular approach is also possible, toward a π-hole that is present above the X-C bond. MP2/aug-cc-pVDZ calculations indicate the latter geometry is favored for X = F, and the σ-hole structure is preferred for the heavier halogens. The π-hole structure is stabilized by charge transfer from the NH3 lone pair into the π*(CN) antibonding orbital, and is characterized by a bond path from the N of NH3 to the C atom of XCN, a form of tetrel bond. The most stable 2 : 1 NH3/XCN heterotrimer for X = F and Cl is cyclic and contains a tetrel bond augmented by a pair of NHN H-bonds. For X = Br and I, the favored trimer is noncyclic, stabilized by a tetrel and a halogen bond.