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 ΔGbind. 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 ΔGbind values, ranging from −8 to −10 kcal/mol. The study paves the way for a green, recyclable, and adjustable SO2 absorption method.