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 SO2-, indicating that the SO ligand should be considered as SO2-. 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).