Bidentate SO₂ Complexes of Zirconium and Hafnium Difluorides with Highly Activated S-O Bonds

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 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 SO₂^2-, which is consistent with the rather long S-O bond lengths (∼1.7 Å) as a result of the two-electron transfer from the metal center to the 1π* orbital of SO₂ upon formation of the complexes.