Side-On OMoF₂(η²-SO) and OWF₂(η²-SO) Complexes Featuring Peroxo-Like Sulfur Monoxide Ligand

Peroxo-like sulfur monoxide complexes of molybdenum and tungsten oxyfluorides [OMF₂(η²-SO)] were prepared via the reactions of molybdenum and tungsten atoms and SO₂F₂ in cryogenic matrixes. On the basis of the infrared spectra and density functional theory calculations, the SO ligand is bound to the metal center in a side-on fashion, and both complexes possess closed shell singlet ground states. The experimental S-O stretching frequencies of OMoF₂(η²-SO) and OWF₂(η²-SO) are much lower than those of SO^- but close to that of singlet SO^2-, indicating that the SO ligand should be considered as SO^2-. This is consistent with the rather long S-O bond length in comparison to that of the ONbF₂(η²-SO) and OTaF₂(η²-SO) complexes with SO^- ligand. Bonding analysis results reveal that the π* (3π) orbitals of the triplet SO molecule are both doubly occupied in OMoF₂(η²-SO) and OWF₂(η²-SO) due to the two-electron transfer from metal to SO. End-on isomers with M-OS or M-SO geometries were also predicted to be stable, but all of them are higher in energy than the side-on complexes, and the calculated frequencies are inconsistent with the experimental values. Comparisons in bond lengths, vibrational frequencies, and natural charges between the OMF₂(η²-SO) and OMF₂(η²-O₂) complexes (M = Mo, W, Nb, Ta) further confirm the peroxo-like character of the SO ligand in OMoF₂(η²-SO) and OWF₂(η²-SO).