When the Deepest σ-Hole Meets the Strongest Nucleophile and Ignores It: Chalcogen Bonding in Polyfluoroaryl Tellurium(IV) Dihalides

A design rule underpins much of σ-hole crystal engineering: deepen the σ-hole (in terms of Vs,max), pair it with the most nucleophilic site available, and the strongest synthon follows. We report polyfluoroaryl tellurium(IV) dihalides, ArF2TeX2 (ArF = perfluoro-4-tolyl, 4-tetrafluoropyridyl, and perfluorobiphenyl-4-yl; X = Cl and Br), in which this rule fails. Four crystal structures show Ψ-trigonal-bipyramidal tellurium with both σ-holes engaged in Te···X chalcogen bonds, forming one-dimensional chains. In PyF2TeBr2, the global molecular electrostatic potential minimum lies on the pyridyl nitrogen (−15.5 kcal/mol), far below the halogen values (−8.3 to −11.6 kcal/mol), yet only Te···Br contacts form. A hypothetical Te···N associate, relaxed at the same level, remains less stable (−8.6 kcal/mol) than either the observed Te···Br dimer (−8.8 and −9.5 kcal/mol) even at its own optimum, settling at a poorly aligned ∠C−Te···N angle of 157.6° that limits orbital donation to 3.1 kcal/mol against 7.1−7.9 kcal/mol for Te···Br. The obstacle is steric and self-inflicted: the parent telluride PyF2Te engages both σ-holes through Te···N contacts, but the bromine ligands that deepen the σ-hole also screen the approach to it, the nitrogen itself being flanked by ortho-fluorines. Acceptor accessibility, not electrostatic depth, governs the outcome.