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

摘要

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 p-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 sigma-hole structure is preferred for the heavier halogens. The pi-hole structure is stabilized by charge transfer from the NH3 lone pair into the pi*(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 center dot center dot center dot N H-bonds. For X = Br and I, the favored trimer is noncyclic, stabilized by a tetrel and a halogen bond.