Unveiling Unusual Reactivity of SO₂ and Unusual Type of S-X Long Bonds

A new reactivity of SO₂ to form unusual stable M-X-SO₂ (X = F, Cl, H) complexes is unveiled in this study. Moreover, a new type of S-X long bonds, which are significantly longer than traditional S-X covalent bonds, has been discovered. The P,N-ligated Ni-F complex model 1A can bind a SO₂ molecule through the new F-S long bond (2.207 Å), and a stable Ni-F-SO₂ complex 1B is generated, being exergonic by 2.2 kcal/mol. According to natural localized molecular orbital analysis, the new S-F long bond has a unique p(F) → π*(O═S═O) bonding interaction, which is shown to arise from the long S-F length. In comparison, the strength of the new F-S long bond (-2.2 kcal/mol) is found to be significantly stronger than common noncovalent interactions such as the hydrogen and halogen bond. The substituent modulations suggest that the electron-donating groups can increase the strength of new F-S bonds and enhance binding free energies ΔG_bind. The scope of possible M-X complexes was explored, and various metals and X (F, Cl, and H) ligands were found to form stable M-X-SO₂ complexes. Specifically, the anionic M-X complexes display much higher ΔG_bind values, ranging from -8 to -10 kcal/mol. The study paves the way for a green, recyclable, and adjustable SO₂ absorption method.