Physical Chemistry Chemical Physics
Royal Society of Chemistry
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 NH∙∙N H-bonds. For X=Br and I, the favored trimer is noncyclic, stabilized by a tetrel and a halogen bond.
Scheiner, Steve and Nziko, Vincent de Paul, "Comparison of π-hole tetrel bonding with σ-hole halogen bonds in complexes of XCN (X = F, Cl, Br, I) and NH3" (2016). Chemistry and Biochemistry Faculty Publications. Paper 689.