Transition metal-SO2 complexes with different S-O bond lengths serve as ideal models for the understanding of catalytic activation of SO2 at different stages. Herein, sulfur dioxide complexes of zirconium and hafnium difluorides [MF2(O2S)] with highly activated S-O bonds were prepared in cryogenic matrixes. The structures of these complexes were identified by infrared spectroscopy and density functional theory calculations. Both ZrF2(O2S) and HfF2(O2S) were predicted to have singlet ground states and nonplanar C-s geometries with the metal center coordinated by both oxygens of SO2. The much lower O-S-O stretching vibrational frequencies (650-730 cm(-1)) in the ligated complexes indicate that the SO2 ligand should be considered as a singlet SO22-, which is consistent with the rather long S-O bond lengths (similar to 1.7 angstrom) as a result of the two-electron transfer from the metal center to the 1 pi* orbital of SO2 upon formation of the complexes.