Department of Chemistry – BMC, Uppsala University, Husargatan 3, Uppsala 752 37 Sweden.; Department of Chemistry, Norwegian University of Science and Technology, Høgskoleringen 5, Trondheim 7491 Norway ; Centre for Material Sciences and Nanotechnology, University of Oslo, Sem Sælands vei 26, 0371 Oslo Norway ; Department of Chemistry – Ångström Laboratory, Uppsala University, Lägerhyddsvägen 1, 751 20 Uppsala Sweden
Abstract:
Three-center, four-electron bonds provide unusually strong interactions; however, their nature remains ununderstood. Investigations of the strength, symmetry and the covalent versus electrostatic character of three-center hydrogen bonds have vastly contributed to the understanding of chemical bonding, whereas the assessments of the analogous three-center halogen, chalcogen, tetrel and metallic -type long bonding are still lagging behind. Herein, we disclose the X-ray crystallographic, NMR spectroscopic and computational investigation of three-center, four-electron [D–X–D]+ bonding for a variety of cations (X+ = H+, Li+, Na+, F+, Cl+, Br+, I+, Ag+ and Au+) using a benchmark bidentate model system. Formation of a three-center bond, [D–X–D]+ is accompanied by an at least 30% shortening of the D–X bonds. We introduce a numerical index that correlates symmetry to the ionic size and the electron affinity of the central cation, X+. Providing an improved understanding of the fundamental factors determining bond symmetry on a comprehensive level is expected to facilitate future developments and applications of secondary bonding and hypervalent chemistry.The factors determining the symmetry and the fundamental nature of the three-center, four-electron bonds are assessed.
