Peroxo-like sulfur monoxide complexes of molybdenum and tungsten oxyfluorides [OMF 2 (η 2 -SO)] were prepared via the reactions of molybdenum and tungsten atoms and SO 2 F 2 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 sideon fashion, and both complexes possess closed shell singlet ground states. The experimental S−O stretching frequencies of OMoF 2 (η 2 -SO) and OWF 2 (η 2 -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 ONbF 2 (η 2 -SO) and OTaF 2 (η 2 -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 2 (η 2 -SO) and OWF 2 (η 2 -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 2 (η 2 -SO) and OMF 2 (η 2 -O 2 ) complexes (M = Mo, W, Nb, Ta) further confirm the peroxo-like character of the SO ligand in OMoF 2 (η 2 -SO) and OWF 2 (η 2 -SO).