Chalcogen bonding of two ligands to hypervalent YF4 (Y = S, Se, Te, Po).

The ability of two NH3 ligands to engage in simultaneous chalcogen bonds to a hypervalent YF4 molecule, with Y = S, Se, Te, Po, is assessed via quantum calculations. The complex can take on one of two different geometries. The cis structure places the two ligands adjacent to one another in a pseudo-octahedral geometry, held there by a pair of σ-hole chalcogen bonds. The bases can also lie nearly opposite one another, in a distorted octahedron containing one π-hole and one strained σ-hole bond. The cis geometry is favored for Y = S, while Te, and Po tend toward the trans structure; they are nearly equally stable for Se. In either case, the binding energy rises rapidly with the size of the Y atom, exceeding 30 kcal mol-1 for PoF4.