A Quantitative Molecular Orbital Perspective of the Chalcogen Bond

We have quantum chemically analyzed the structure and stability of archetypal chalcogen‐bonded model complexes D2Ch⋅⋅⋅A− (Ch = O, S, Se, Te; D, A = F, Cl, Br) using relativistic density functional theory at ZORA‐M06/QZ4P. Our purpose is twofold: (i) to compute accurate trends in chalcogen‐bond strength based on a set of consistent data; and (ii) to rationalize these trends in terms of detailed analyses of the bonding mechanism based on quantitative Kohn‐Sham molecular orbital (KS‐MO) theory in combination with a canonical energy decomposition analysis (EDA). At odds with the commonly accepted view of chalcogen bonding as a predominantly electrostatic phenomenon, we find that chalcogen bonds, just as hydrogen and halogen bonds, have a significant covalent character stemming from strong HOMO−LUMO interactions. Besides providing significantly to the bond strength, these orbital interactions are also manifested by the structural distortions they induce as well as the associated charge transfer from A− to D2Ch. Covalency strikes back! Our quantitative molecular orbital analyses show that chalcogen bonds have a considerable covalent component and are far from being purely electrostatic phenomena. They closely resemble halogen bonds and have similarity with hydrogen bonds.