Bidentate Sulfur Dioxide Complexes of Scandium, Yttrium, and Lanthanum Difluorides

The SO₂ complexes of scandium, yttrium, and lanthanum difluorides [MF₂(O₂S)] were prepared via the reactions of laser-ablated metal atoms and SO₂F₂ upon UV-vis irradiation in cryogenic matrixes. The presence of bidentate SO₂ ligand in the products was demonstrated by the characteristic infrared absorptions as well as isotopic frequency ratios from both S¹⁸O₂F₂ and ³⁴SO₂F₂ experiments and is further supported by DFT calculations. All three product molecules were predicted to have nonplanar C_{2 v} symmetry with the SO₂ ligand bound to the metal center through both oxygens. The computed S-O bond length and stretching frequencies of ligated SO₂ approach those of SO₂^- as a result of electron transfer from metal center to the 1π* orbital of SO₂, in agreement with the results from bonding analysis. On the basis of DFT calculations, fluorine transfer from SO₂F₂ to the metal center to form the MF₂(O₂S) complexes is highly exothermic. Although a proposed intermediate in the form of MF(O₂SF) was predicted to be stable, it was not observed in the experiments, presumably because of the low energy barrier for further isomerization to MF₂(O₂S).