Sulfur Dioxide Complexes of Main-Group Elements: from SO20 to SO2- and SO22- upon Coordination to Aluminum and Silicon Difluorides.

The AlF2(O2S) and SiF2(O2S) complexes with bidentately coordinated sulfur dioxide ligands were prepared via the reactions of aluminum and silicon atoms with SO2F2 under UV-vis irradiation in cryogenic matrixes. The products were identified by infrared spectroscopy as well as DFT frequency calculations, and nonplanar C2v structures with the SO2 ligand bound to Al/Si via two oxygen atoms were established. In spite of the similarity in geometric structures, the electronic structures of AlF2(O2S) and SiF2(O2S) are completely different. The longer S-O bond length and the lower O-S-O stretching vibrational frequencies of SiF2(O2S) in comparison to those of AlF2(O2S) indicate that SiF2(O2S) possesses a singlet SO22--like ligand while the SO2 ligand is better described as SO2- in AlF2(O2S), which is further confirmed by bonding and population analysis. Linear correlations were established between the experimental O-S-O stretching frequencies and calculated S-O bond lengths of SO2, AlF2(O2S), and SiF2(O2S), suggesting that the S-O bond is gradually activated when SO2 is coordinated to AlF2 and SiF2.