Side-On OMoF(η-SO) and OWF(η-SO) Complexes Featuring Peroxo-Like Sulfur Monoxide Ligand.

Peroxo-like sulfur monoxide complexes of molybdenum and tungsten oxyfluorides [OMF(η-SO)] were prepared via the reactions of molybdenum and tungsten atoms and SOF 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 OMoF(η-SO) and OWF(η-SO) are much lower than those of SO but close to that of singlet SO, indicating that the SO ligand should be considered as SO. This is consistent with the rather long S-O bond length in comparison to that of the ONbF(η-SO) and OTaF(η-SO) complexes with SO ligand. Bonding analysis results reveal that the π* (3π) orbitals of the triplet SO molecule are both doubly occupied in OMoF(η-SO) and OWF(η-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 OMF(η-SO) and OMF(η-O) complexes (M = Mo, W, Nb, Ta) further confirm the peroxo-like character of the SO ligand in OMoF(η-SO) and OWF(η-SO).